Assuming that running Windows is a non-issue, the parts that really, really bug me are:
- Background operations suddenly become foreground operations that stall the IDE with a dialog saying "A background operation is taking too long."
- Misclicks spinning off long background tasks that stall the IDE.
- Automatic refactoring/code analysis that will stall the IDE.
- Incredibly aggressive automatic reformatting.
- The Microsoft Account agenda. VS2013 wants me to sign-in with my Microsoft account on so many different occasions: settings sync, Azure, Windows Developer account registration, auth refresh, etc... which wouldn't be so bad, but I'm signed in via Windows 8 already, adding insult to injury.