Ok, from biggest to smallest. And don't start to cry:
- horrible collision detection
- horrible ASF physics
- view distances in general (limit should lie above 3km to prevent all the ugly mess ups)
- lack of cockpits, 'Mechlab, effects to visualize damage
- lag issues, hit-detection problems
Most of that will never get any better, that is the reason why MWO is potentially attractive to me.
Don't get me wrong. I love MWLL and I'll try to stick with it. But as I said: The overall better product will get all of my attention.
I'll respond to these just because its good to be clear on things. I will point out that none of these are really bugs btw

Collision detection in what sense? Hits, or mech to mech? This is a bit vague sorry, but its about as good as you can get (I don't think MWO will do any different), but maybe I'm not understanding you.
I'm also not sure what you mean by horrible ASF physics? Are there bugs at this point? If you play BF3 its quite similar if not better.
Again with view distances that's a performance issue. No game is going to give you this quality and have those sort of view distances. You mean on mechs, objects, or what?
Cockpits are defintely something that we can't hit without about 3 more artists dedicated to it. Just a fact of life, and it will be cool to see how far MWO can take them.
Hit detection is an ongoing thing, but it will never be perfect. It isn't in any game you play, and we opt for the version thats most fair on average for everyone (even if it means that sometimes you don't get your shot). Hopefully with a few more server side code improvements, responsiveness should increase and thus decrease lag.