That's not their real new logo. They are doing "30 days of change", where they use a different logo each day, leading up to the real on one on Sept 5. The OP is the day 24 logo.
I'm pretty sure that this is just temporary as it's part of Yahoo's Thirty Days of Change (http://www.yahoo.com/dailylogo).
Better than the current one, but not really that great? It feels like they're riffing on the new iOS 7 style but not quite pulling it off...
The original logo reflected fonts from early 2000 and the new one is light weight font,Microsoft started using it few years back before Apple adopted it.
83,377 opinions, 24 new Yahoo! logos. Only 1 has beaten the original: