I was a beta tester several times, and I have to congratulate Zenimax on the launch. Though it had a few problems at the start, comparing the beta tests to now the game has made a huge improvement.
Onto a suggestion:
Way too often I'll be in a party, we'll load into an area and become invisible to one another (which the beta also suffered from). I know having instances of the worlds helps take the stress off the servers, but it'd be nice to allow us to choose the instance/server we're on, which is what a lot of MMOs have done. It's hard to enjoy groups when you're having to frequently re-log just to make the members reappear.