Aesthetically I like it, but I agree about the contrast. It's hard to tell the difference between a greyed out button and an active one. I'll need to think up something to write, and a few other things to do to be able to really form an opinion on it, and even then it'll only be on Word and Outlook, as I don't use anything else regularly.
I've been giving both Word and Outlook a proper shakedown because, like you, they are the two programs of Office I use the most. No minuses, and there are a few pluses.
1. Outlook 2013 is faster than 2010 - no matter which way you slice it.
2. Word 2013 now edits PDFs natively - in short, it treats PDFs (and likely ODF files as well) on par with Word documents. Is this why Flash is now included with Windows 8 - compensation for pilfering Adobe revenue? (This is actually understandable- which is a business - any business - more likely to have already - Acrobat or Word? And if I were in Adobe's shoes, I'd certainly want quid pro quo for all that revenue I'm no longer going to get.)
Those two reasons *alone* are plenty to keep Office 2013 around.