Facebook Concept
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I am actually not a fan of Facebook but I somewhat like this concept. http://www.behance.net/gallery/Facebook-New-Look-Concept/6504647

It almost reminds me of Metro when you get to the dashboard grid layout.
A3
Looks sleek, but I'm wary about the grid design. It's a visual cluster of information and makes it almost too easy to weave advertisements into it.
Obama_making_a_decision
How horrifying.
1269375234858
Besides the grid the rest of it is actually good.
Avatar
Very pretty, but the usability of that site sucks. Seriously, you don't know which order to parse information and the second you stick photos that aren't perfect in there it's not going to look anywhere near as good.
Acehuge
This is a horrible UI. There's no focal point as you view the homepage, there's no clear direction as you continue (I mean, this is 101: http://www.nngroup.com/articles/f-shaped-pattern-reading-web-content/). There's no whitespace, making the whole thing look incredibly busy. It uses horizontal scroll. It's just so bad.
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Alain said:
... the second you stick photos that aren't perfect in there it's not going to look anywhere near as good.

This was the first thing that came to mind. Your average Facebook user has just a point and shoot camera (or iPhone) and no idea how to photography. It'd be a mess of blurry low res photos.
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I don't like the grid layout at all.