One thing I'm really going to miss: being unable to recreate certain characters "perfectly".
My first character, Tau Zero, was still using costume pieces and a face that were no longer available
(having been replaced by later skins).
For quite a few years, he could never go to Icon. But since he had the "perfect" costume that didn't matter.
I'll miss my vet rewards, I'll miss my base (a lot), and I'll miss all the "other" progress I had in the game,
but I'll miss Tau's old costume most of all.
Still, I'll happily give it all up just to be playing again.