I'll start (in a post separate from the OP so it can remain separate and clear):
One thing I've always wanted to praise to whoever is responsible: How LRMs work. This is the first MechWarrior where the minimum range thing feels perfect - aside from the fact they won't arm for IS, the upwards/downward arc even for Clan LRMs does a perfect job of making them feel just right. They were really nailed perfectly.
Second, I love the fact extra care was given to including a lot of electronic packages. This was one of the best things about 4, and they did a great job of blending the mechanics of 4 with CBT here giving many more options: ECM, GECM, AECM, BAP, BHP, Enhanced Optics and even a great take on C3! This really lets units have different grades of options and gives them more unique roles and lets the specialty units really shine.
And one more for now: Being the first MechWarrior game to let me pilot a hovercraft in multiplayer. I've always loved hovercraft in CBT, but until now, the only instance of getting to actually drive one was a single player mission in MW2: Mercs. For the first time they are not only in MechWarrior, but perfectly balanced into the game. While they started out terribly underpowered in the first release, they're now highly effective backstabbers and raiders. Major props.