I prefer a lot of what they did with MW:4 mercenaries, specifically the balancing of weapons.
All previous mechwarrior games simply translated the stats straight from the board game and then let you shoot them much faster than once per 10 seconds like it is in the board game. This made ER Medium Lasers the most powerful weapons in the game. By adjusting the damage of the weapons based on board game stats and then modified by refire rates, they made previously unused weapons (like the AC/5) a dangerous weapon.
But, all mech games to date have suffered from the same problem and have failed to address it, and that is the pinpoint accuracy of an alpha strike. The random hit chart from the board game has a lot to do with the survivability of a battlemech, but in the mechwarrior games if you point and click at the center torso, then you dump everything into the center torso. MW4 alleviated some of the instant death by making it impossible to kill a mech with one shot (need at least 2), but it still does little to solve the problem of jump snipers armed to the teeth with ER Large Lasers and Gauss Rifles.
Mechwarrior 4 is at the head of the class in my book, despite the step back in graphics from the grittier MW3. You can play with 24 people in a server, and that's some serious mech action. Can't wait for MW:LL with 32 total.