New rendering engine is webkit which is used in Chrome and Safari.
You must have joined pretty late in the beta because early on it was UNUSABLE. When the beta first hit it was probably one of the worst GUIs I've ever seen but they've cleaned it up A LOT since then made it better than the old one.
I was using the beta since day 2 of it. and I know of all the problems it faced in the begining with the messages poping up with no text or all like //////////// and all the other crap. But I kept with it mainly because I love the list view, with the backdrop changing based on game with all the info there. I really hate the generic buttons though we don't need some sort of iphone app list. Also the ability to find all the steam groups associated with games and whatever. It's very nice.