Forgive the mildly long reply, but it's not ranting or anything - just a thought on each of the topics that you suggested, Herb.
Content: I like seeing the all-encompassing TROs, especially the way the last few have come out. Certainly BattleMechs are important, but I think a little more focus on modern vehicles, BA, and fighters would do well. Round out the content a little more, I think, especially since we're told that tanks and whatnot are easier to produce. I dunno if historical units would engender their own TRO separately (IIRC that's being done for the Star League, but I could be wrong) however an "Oldies But Goodies" section would be cool to help flesh out areas we don't see at present - something like 3075's historical bit. Maybe even introduce new infantry weapons that way, too, like 3026 and 2750 had their personal gear sections. Who knows, it might increase sales of ATOW!
Writing: LOVE all the background details, plot hints, notable pilots, and whatnot. Developmental histories, political shenanigans (see also Lyran Stealth Armor) all really flesh out the universe with a small investment in wording...but admittedly a big investment in factchecking. Especially without the novels, it's the best way to introduce characters and events, IMO. Deployment data is really useful for someone like me; I love factional-flavoring and try to keep the feel of a regiment consistent with the fluff. Phyr's Hussars likes Dolas? Sweet! Lots of DOL-1A*s for them when I make their unit.
Stats: I'm happy with all the stats. I admit I'd like to see location placement for things like Endo Steel, Heat Sinks, and such simply because that lets me use units that don't yet have record sheets (see also TRO Prototypes) because I can make them myself accurate to canon. I don't need BV or Cost, personally, though I find it a useful bit of side-data sometimes. LOVE having quirks around, please don't drop them! The suggestion of QS stats is very cool too. Would it be possible to include RPG stats for battlesuits at least, or would that be too much to ask for?
Rules: Definitely would like to see more 'experimental' and 'advanced' TL stuff. I'd like to see some more tie-in (as above) with the RPG, at least stat sheets or something. I know there's conversion charts, but I admit I'm lazy and wouldn't mind having official stats.
Art: ERHMAHGERD you guys have been awesome and my only request is more! I also would like to see further 'visual thematics' applied - stuff like the instantly-recognizeable look of the Celestials, or of the classic Clan omnis, or the Star League's curvy armor. We've got different factions, why not give each of their production a bit more visual cue to identify them? Doesn't have to be major, but say...bird legs are popular in the Combine, satyr legs are popular in the Confederation, barrel chests and gun-arms are popular in the Suns, that sort of thing.
Layout: Not much to say here. I admit I like the separation by type more than I do by "side" - having all the Clan tanks with the IS tanks in their own TANK section, rather than "these are all IS units" and "these are all Clan units" works better for me; it's a pan-humanity thing. I WOULD like to see a lot less grey in the readout layout - it makes it hideously expensive to print my PDFs and arrange them the way I like, because the text gets all kind of background on it that annihilates my ink. It might also save on costs since you wouldn't be using as much, so perhaps a bit more white space?
That's all, please don't delete me! I know it's long but I'm throwing ideas, not ranting!