The only major issue so far with Win 7 (x64) is Internet Explorer 8. It can best be described as "not ready". The reliability monitor in Win 7 agrees with me. It gives my system a reliability ratings on 1 - 2 out of 10 due to IE.
9 of out 14 error reports send so far is caused by IE (4 days period).
The version of IE that comes with win 7 is 8.0.7000.0 and the version that Microsoft released as RC1 on in December is version 8.0.6001.18343
If that version number means that the version of IE that ships with Win 7 are a later build the RC 1 Microsoft will have a very huge problem when IE 8 goes RTM.