I agree with everything intooblivion said, but must also add to the list, PERSONAL E-PENIS METER!
(aka, gamerscore) What’s the point of achievements, if you can’t see a brag-able number displaying the exact number of inches your e-peen has grown because of it!? But in all seriousness, I wouldn’t want to see a giant virtual penis growing with my every move in my games. What I’m trying to say is, that we need a number for our achievements, why not copy the xbox all the way!?
Anyways, besides this strange talk of e-peens and such, I have had performance problems as well, it appears that they haven’t ironed the kinks out just yet, (hence the beta status) and will hopefully fix any performance problems, maybe making it faster. Other than that, I feel the downscaling quality of the grid view, should be drastically improved (it just looks terrible when your not zoomed in completely, which makes scrolling through games a long and annoying task), and the list view should allow for custom banners for games (mostly for mods or non-steam games) considering that a plain exe icon isn’t going to cut it.
Lastly, this seems to be a step in the right direction in terms of flashiness, (it doesn’t seem to really be adding that many important features) but I’ll always miss the simple look of classic steam. I highly doubt any “classic” skin will ever be released, so enjoy it while it lasts. 
PS: Just remembered this but am too lazy to edit e-peen rant to include, they should also add FULL COLOR manuals for ALL GAMES, that are viewable from INSIDE STEAM! Anyways, good night. I hope my rant may have affected you in some way