It's missing some of the gesture customization others have. I particularly like the left AND right swipe gestures in Thunder. Plus, there are more actions you can assign to them.

Thunder also has more visual adjustments. Things like edge to edge images and post action customizations.

Also, the reply window makes formatting and quoting easier.

The feature different isn't big though, and most of them aren't a big deal.

I'm not sure why you think Thunder is ugly though. The way I have them setup, they look almost exactly the same, except I have nested comments in factors more visible on Thunder, which makes it a bit easier to track the conversation.