On the subject of 95% of the players 95% of the time, I'm only going to talk about my own experiences, and how I have used each of the books. In order to help to make the best product possible, I need to offer criticism on what I think needed work. I don't want to poo-poo on Catalyst or it's authors.
CAVEAT: Don't mind the Battlemaster. I am speaking as a player. I have no more authority over things than any other run-of-the-mill player. Being demo team just makes things easier to help teach new players how to play.
First of all, I do not do campaign games. Campaigns work when you have a venue that is your own. Probably your own house. You have to wrangle cats to get everyone in the same spot, and as soon as one of those cats starts getting bored, the whole thing collapses in on itself. The GM always has to be there to run the other guys through the game, and play mooks, rather than allowing each player to fight other players on their own time. Combine that with all the crunch in the standard Battletech campaign system (Not to say I'm not enamoured by the idea, but I'd have an easier time finding Sasquatch than someone to be my GM for a Battletech Campaign) and it really does make more sense for me to stick to pick-up games at an LGS. That has been the play style I have used most frequently. Scenario games that I run at cons do use a lot more content from the other books, but those are for planning, prepared ahead of time at home, not made up on the spot. The things I need when running the scenarios have proven to be the same, or less, than for pick-up games.
Tech Level:
I love keeping up with the latest tech, but for every pick-up-game where I get to play with the newest toys, there are another ten where I'm getting shoehorned into using tech from the clan invasion or earlier. This is either due to players' inexperience with the game and needing to be weaned into it at introbox level, or because they're the grognardy-type who bought things 20 years ago and they'll be explitive verbed if they ever have to spend money again. Lots of Introductory, A good amount of Tournament Level, Advanced/Experimental, besides artillery, I've never used in a tabletop game. I will use any Adv/Exp gear introduced in this rulebook, though. It's more about not having to lug extra weight or concede to extra rule use.
Total Warfare: Most of the rules come into play in Total Warfare, though buildings have often been treated as impenetrable concrete bricks. Most of melee gets used, too, but I have never seen anyone use a club in the game - ever. Neither tree, girder, or mech limb. I have never seen anyone use Semi-Guided LRMs, but that might be in part because many people still think that TAG counts as spotting for indirect fire, and it's not explicitly said well enough that spotting and designating are different things, making the extra BV costs for SG LRMs not worth it to them. Also Missile-related? One Shot launchers have been all but unused for years due to inefficiency, so most players don't realise that they can't explode from crit hits or overheating. Being clear about that is important. C3 Networks had small wording changes that made huge changes on gameplay. For a while, C3 could be spotted without LOS between the closest mech and the target (Attacker needs LOS though). This was almost enough to make C3 worth the brutal BV costs. Later, it was written back to requiring LOS for the closest mech. It really makes C3 little more than a white elephant in a BV-based pickup game, that way. Industrial Mechs never got used, nor the equipment that they usually carry. All the Combines and Chainsaws and Mining Drills were dead weight. Same can be said of Support Vehicles, and all the Aerospace stuff. VTOL is OK, but there aren't enough VTOL in the game that have minis for it to have ever come into play. WiGE rules I still haven't used, and they're kind of tough to wrap my head around. Speaking of unused things: Rail. There are no train minis. There are no maps with traintracks. Not sure why this wasn't TacOps'd
I did not like the inclusion of Miniatures assembly and painting tips, as it really isn't something I need at an LGS, and doesn't do much but add page-count. It should be reserved for an introbox, website, or miniatures rules book.
The mission types in the "Creating Scenarios" was nice, but some need more clarification or to be reworked. Most of the scenarios sound like "Kill'em all" with differing start points. I'd love to use hidden units rules, but when both sides' objective is "kill'em all", I can't see justification for the defenders needing to have half the BV of the attackers, even if they can hide some of their units.
I love RATs both as a means to have randomly selected forces, and as a means to know what kind of mechs each side should have, but I think greater care is needed in balancing the quality of mechs in each RAT. In all honesty, though, RATs don't see use in my games, because it means you don't know in advance what it is you need to bring to the LGS in terms of record sheets, and unless you're buying dozens of each, you don't know how many of each mini you need before you're forced to proxy. RATs are fantastic as part of campaigns and RPGs, and I think it's a must have. As part of a pickup game away from the comforts and resources of home? Never.
There are a lot of people who still prefer older editions of BMR over Total Warfare, but most of those complaints are about layout and difficulty of finding rules. I'm inclined to agree that TW's layout and ease of use is lacking, but I LOVED a lot of the changes that were made in TW. Partial cover not being a handicap? Falling in water doing half-damage? Machine Guns and Flamers being useful at anti-infantry work? Superb. Vees, BA, and Infantry being made viable alternatives oh my yes. The index, however... That was the biggest issue. I remember finding no entry for Improved Jump Jets, but for some reason, there was an entry for Pin Vice.
Lastly... Personal taste... I really prefer to see artwork in books. Illustrations, especially full color ones, are WAY more exciting and dynamic to me than pictures of minis. Later rulebooks get the hang of minis dioramas that look interesting, but much of Total Warfare looked like snapshots of any old miniatures-rules pickup game and didn't feel exciting or dynamic enough. By TacOps and beyond, the dioramas started looking like real dioramas, but all things considered, I'd still rather see some art from Alex Iglesias or Anthony Scroggins in there.
TechManual: I'll be honest, save for calculating the BV of a force, this book has been almost completely unused by me. Checking the production year of some equipment was handy, but it's otherwise meh. We've had mech editing programs for years, and it's way more convenient and clean-looking than churning record sheets off by hand. Further, it's too time consuming to be doing that before a pick-up game. I am glad that they put the construction rules in their own book. That's another 10-50 pages (Depending on how many unit types and level of equipment) of unused rules I don't need to lug around. Calculating how much BV a force costs, though? HUGELY important for playing pick-up games. There was a short, "blink and you miss it" blurb in Total Warfare that a lance should be about 6000 BV, but clearly stating some "Standard BV Values" for players who are trying to build their first mech lance/star for use at an LGS would save them a lot of grief. Scaling these values (or tips for scaling these values) to make play between IS and Clans faster and easier would be a good idea. I'd also suggest some BV tweaks/errata for C3 networks, while we're at it. A C3 lance isn't good. A Level II isn't great, but at least C3i is less fragile. A C3 company is just outright brokenly bad and why I hate double-master mechs with a passion.
Reminding players that Clan mechs' record sheets are pre-calculated as though they had a 4/5 mechwarrior, and not a clan-standard 3/4 mechwarrior, would get me out of some unpleasant conversations I had in the past.
One thing not included in Techmanual/TotalWarfare that should be included: Game-organising etiquette. Clarification that whether or not custom mechs are expected to be permissible for use in a pick-up game without prior agreement. Whether or not the team behind Battletech want to put their foot down on whether or not custom mechs are tournament legal is fine, but more importantly is what is to be expected as the default game. For example, if I show up to a pickup game with a stock lance of mechs with 3025 tech, I don't think anyone's going to complain, but if all of those mechs are min-maxed custom builds using Jihad-era Tournament-level tech, I'm pretty sure that some folks would take issue with having to fight against it with the stock 'mech record sheets they printed off. On that note, with the fluff for Omnimechs, are players allowed to customize the pod space however they like before a match without asking, or is the default to stick to the canon configurations? Likewise, if someone has a star of clan mechs at 15000 BV, and the other guy only shows up with a lance of 3025-mechs valued at 5000 BV... Different people have different tastes, and agreeing in advance on things is the best way to go, but having a clear setup of "If we didn't talk it out before the game, what are the guidelines I should follow for preparing for a pickup game" should be listed.
Tactical Operations: I wanted to use this book more often, but it was a whole lot of book for something that I would need to pick and choose rules from. I felt it better to just say "Total warfare, final destination" than having players pick and choose rules that benefit only them. I have had bad experiences with hells horses players refusing to play unless enhanced vehicle survivability rules were in play. I wouldn't mind rules like those if they came with positives and negatives, but outright improving something with no drawbacks might as well be throwing BV out the window. Having the equipment would be nice, but the extra rules are just points of contention and aren't really needed.
In theory, variable weather sounds fun and awesome. In practice, pretty much everything means "your shooting is even worse, now". Good for adding flavor to scenario games, but overall seems like overkill when TNs are often already at 10+ with regular warriors.
Strategic Operations: Only for Campaigns. I felt like the Repair/Refit/Customization rules in here should have been included in Techmanual, because so many players treat mechs like nothing but a mass of lego bricks. If my suggestion for not having construction rules in BMM goes ignored, at least include the Refit/Customization rules, too.
Interstellar Ops: Some of the era-related tech was interesting and could be handy for sculpting scenario games, but again, nothing there is something I need for a pick-up game.
Campaign Operations For campaigns. No duh. That being said, if you guys want to beat me to building a simple, No-GM, come-as-you-please pickup game campaign system that allows players to use whatever minis they have/want and can be taped to and tracked on the wall of an LGS (and no, not the whole wall), I'm all for it being included.