There have been a lot of things - being able to tailor a mish diff was huge, as was the team-levelling thing. But yeah, the big thing was the ability to change the colors of the powers - being able to set the powers to match the costume instead of the reverse was such a change.
Disabling the profanity filter.
That #$%@$ changed the game for me.
AV -> EB feature - ZOMG, now I can get to maximum level with certain heroes and villains, while still doing my story arcs!
Mission auto-complete feature - ZOMG, now I can get to 50 with any character, while still doing the story arcs! The gates have been fully opened!
Those two were easily the most important changes for me. There were many other cool changes, but they tend to pale in comparison.
Power Customization was basically a tactical nuke for my brain. One of my oldest characters, Emerald Dervish, is supposed to be made of sand. Since the best thing I had was Dark Armor to show this, I just said the sand was black, like volcanic sand. I've long since considered making it tan, but it's become such an integral part of his look I can't do it.
Lesse...Costume change emotes, that made RPing transformations so much easier. I used to type out detailed "/e" descriptions of a transformation, now I just use /cce ccPureEnergy and call it a day. :P
But the biggest, earth-shattering change I've seen in my career? Inherent Fitness. It still blows my mind that we get these things free now. It was like the floodgates opened, and millions of possible builds came pouring out. All that meticulous thought over which three powers you were going to sacrifice for Sprint, Health, and Stamina? Yeah, no more of that.
The off-beat space pirate...Capt. Stormrider (50+3 Elec/Storm Science Corruptor)
The mysterious Djinn...Emerald Dervish (50+1 DB/DA Magic Stalker)
The psychotic inventor...Dollmaster (50 Bot/FF Tech Mastermind)
Virtue Forever.