Only pleasant surprises for me... and those were minor since I researched and test drove my 550i GT thoroughly before ordering it. The only things that come to mind:
First, turning radius is surprisingly tight, at least with the new Active Steering. My 2008 650i convertible had an earlier generation of it but could not make many U-turns in my neighborhood. The GT, which is longer, can make all of them without a problem.
Second, driving at night is somehow easier. The mirrors in my 650i would almost black out at night, making it hard to see anything more than headlights of cars behind and around me. In the GT, I can see much more of the road and car bodies than before. This may be due to being higher up off the road (the 6 is especially low, and the GT sits higher than most sedans) or better mirrors or both.
I thought I had a third point it's escaped me. If I think of it I will post it later.
I also considered one of the new X3's... ultimately it felt a little too stripped-down for me, though it's obviously a huge step up from last year's model. I just wanted a more luxurious experience, and the GT gave me almost as much as a 7 Series, for about $20k less... overall very happy after a few months with it.