1) I might find it more useful if it was limited to tweets from the people I follow or where the votes were from people I follow. I know that is much more difficult data and that it requires much greater adoption to be viable. But I think that would make it useful for me. (you could also use favorites as votes if you can get the data).

2) The design and performance seemed fine. The icons seem less harsh on twitter.com because the row height is closer to the icon size. Also, maybe only display the instructions the first time I visit the page. Go from Popular to Recent and back to Popular. The instructions get in your way.

3) I guess I rolled this one into the first answer.

Now, it sounds like I may want different things from twitter than your target market since for me it is more about ambient relationships than insightful commentary.

Congrats on your launch!