I'm really happy with Visual Studio 2012, and I do think it's by far the biggest leap forward for any iteration of VS. However, one new feature that bugs me is the "Auto Preview" where any file I click on in the Solution Explorer is opened for me in a special preview tab.
This can be prevented if you hold down the ALT-key while clicking around the Solution Explorer, but IMO it should have been the other way round (i.e. don't preview unless you ALT-click).
Luckily this can be easily turned off by going to Tools -> Options -> Tabs and Windows and de-selecting the Solution Explorer under 'Preview Tab'