In Mechwarrior 3 there is no such thing as:
ECM blocking missle locks
Poptarting
pinpoint accurate AutoCannon spam
PPC boating
srm spam
The walking animation is better than what we have now in MWO! Mechwarrior 3 was not a perfect game however and did have LRMS that all hit one section of the mech, but this was due to the lack of computing power at the time. but LRMS were dodgeable if you moved 65+ KPH
The best part of the game was that heat actually had an effect, you slowed down, shut down at 70% (without override), and alpha striking with four ppcs shut you down, 5 or more and you explode
If MWO had gameplay mechanics of MechWarrior 3 combined with today's computing power and the smooth controls of MWO, MechWarrior Online would be out of beta by now. We would have a balanced, strategic, and fun game!
Go play MW3 for yourself and look for yourselves. Please don't get angry at the rather dumb (but competent with heavy/assaults) AI

P.M. Me if you are serious about wanting to get MechWarrior 3 to work with Windows 7, I could only guess with XP and Vista.