House rules that are going to affect the game are a deal breaker. Typically, those that want to use house rules are going to do so for their benefit.
People wanting to use a deck of cards for initiative and everyone pulls a card for each unit is one that someone has tried pushing. If someone has a problem with grasping how initiative works as is, then adding in random card pulls that force a player to move a specific unit is going to bite them in the butt.
People that want to play "mech only" games are seeking to remove elements that they have a problem dealing with.
Some want the games to be on a single map so that their assault mechs can avoid being shot up at range as they crawl forward.
"I hate the clans, so they don't exist!" or "3025 only!" limits the game to what they are comfortable with. Its one thing to have 3025 games here and there, but removing the vast majority of BT's existence is anything but helpful since even the clan invasion was a long time ago in RL time.
Ultra ER Rac20s and other nonsense is ...well..nonsense.
I have had to help a few friends relearn the game because they "learned" the game from a friend of theirs that added truckloads of house rules to the point that they didn't know what was what. Battletech rules surgery was needed. The house rules always seemed to limit things for that one player's enjoyment.
Me, I'd rather people have a fair game with rules that have been play tested. That is not to say that I agree with all of the rules as they are, but they have been play tested and anyone reading this can go to TW and any of the other core books and find the rules of the game before we meet to play. I can give you a page number of an optional rule or show you in the book/pdf and explain it and you can at least know that I didn't make it up to suit my play style.
Optional rules that have been published help add to the game in a manner that has been playtested. Most of what we have seen has existed since the 1990s. However, even a list of optional rules can be used to manipulate the game to benefit a particular play style over another.
Even a house rule that seems as "fair" as "we always have default pilots that are 2/3s" might sound fine, but the benefit for certain units will be much more significant then others. Lights are basically ruled out of the game as those free pilot upgrades are going to seriously boost the ability of assaults to land GR and Ac20 rounds. When I went over that with a visiting player once and it really sunk in that those free pilot upgrades were manipulating the game the look on his face was as though he had seen color for the first time in his life.
Stuff such as movement dice prior to them being published, or even a timer to help reduce excessive time spent per move isn't a bad thing. I have known some that have argued for and against themselves by arguing for and against a timer. "The game takes to long!" but takes 5 solid minutes to move a platoon of infantry, or 10 minutes to talk with their team mate about a single move that they decide not to commit to at the last second and go with what they originally wanted to do is a good start at reducing the time spent per turn. I have played when I only slept 3 broken hours, had to do a lot of stuff before, went drinking the night before, and still take 30 seconds to move, so a timer is not going to affect me. It will affect those that are newer and think out their move as they are about to make them. I'm a nice guy, I'll try to occupy myself as you take a few minutes to consider a move. I'll even slow my moves down as not to spook them as the faster I move units the slower they respond. A movement timer could help people commit to a move and learn through experience instead of holding a conference each move, but I wouldn't want it to be "pick a unit, you have 1 minute to move or its declared as standing still".
Helpful stuff that does not modify the rules is nice, but I wouldn't want to commit to things that are going to punish new or unsure players. This is especially true if they are playing against someone that is going to benefit not only from their lack of experience but them being penalized for it. I have more fun seeing less experienced players learn then I do from beating them to a pulp.
