First, I would like to start off by saying Mechwarrior 2 was definitely my favorite. In fact, the first PC game that I owned (you know, besides family shared games) was Mechwarrior 2. So let me go into the differences between some of the games as I perceived them. I am looking at Mechwarrior 2, Mechwarrior 3, and Mechwarrior 4: Mercenaries.
Mechwarrior 2, I think, was clearly the closest towards being a hard core sim of the 3. You had more control over jump jets, as was mentioned in another thread. You had things like Death From Above (I am not sure if those are in the other 2, but I could never do it. Not sure if that was just my failing

Mechwarrior 3 was fun, but most of what made it fun was the stuff that probably won't apply to MWO. What Mechwarrior 3 had that 2 did not was more interesting characters. The story and maps you got to play for was fun. The ability to salvage from destroyed mechs was good. However, there were things I did not like about mechwarrior 3, and they all would affect MWO. Basically all of the things I listed above were removed, I guess to simplify the experience and make it friendlier to new players. Personally, I think this was a mistake. Now, the simplified the control scheme for 3, which isn't really a bad thing. It sounds like MWO will let you pick how simple your control scheme is which sounds like a great idea to me. The rest of the simplifications made, however, detracted from the overall experience of the game. I am not exactly sure how blowing up when one leg gets destroyed made the game easier for players to understand. Component based damage has got to make a return as well.
Mechwarrior 4, actually, was mostly pretty similar to 3 the way the gameplay mechanics worked so I don't think I have more to add for this one.
To summarize, I think we need the more simulation-style gameplay from Mechwarrior 2, and a choice of either simple controls ala 3 and more sim-like ala 2. Battlefield salvage ala 3 would be interesting to have as well, but I am not sure if that will work with the rest of MWO since we don't have many details on that yet.
What thoughts do you guys have?