Toptube length and geometry, IMO, are the two most important things for mtb fit/comfort. Whether for trail riding, racing, dh/freeride, trials, whatever. Everything else tweaks afterwards. But if you've got either of those two things off from your preferences, you're not going to like the bike. Seat slighlty lower than you'd have it on the road bike as well. Again, all this is just my opinion from starting out as a roadie and having swapped to dirt many years ago.

It all really depends on what sort of terrain you're on as well. Having done a few seasons of racing in the west and the east, the terrain and courses differ, sometimes quite a lot.

Me: based on fit stuff, I ought to be on larger frames. I've come to prefer and be comfortable on much smaller frames for how and what trails I prefer to ride. Enough so that I'm actually more comfortable on smaller sized and slacker geometry road frames now as well. Go figure. 22.5" TT, 15" seattube, short stays, slacker seat and head angles. Yeah, none of my bikes climb like goats because of that, but the handling is the way I want it now on the downs and technical bits. It's a trade off. They can climb anything, they just do it a bit more slowly than a 71* head angled race bike.

Sweeping generalization: as riders start hitting more technical stuff - (not always necessarily steeper) TT and stem lengths shorten up a bit, angles slacken up, and bars get wider. IMO, wide bars also rule. I've yet to have a riding buddy swap to wider bars and decide to go back to narrower.