The changes that have most players up in arms are the nerf to the stun dealt by Total Focus and an animation change to Energy Transfer, changes that hit the Tank, Brute and Stalker classes. The new animation, while more visually impressive than the old, is slower; an Energy Transfer used to take one second, and now takes 2.67 seconds. (For those that don't know, Energy Transfer is a damaging attack that also has a good chance to stun.)
Despite the usual high levels of communication from the developers, some players are unhappy and consider this a crippling nerf; however, nobody seems to be particularly surprised that some form of nerfing has taken place. Energy Melee really was that good.