Dunno
Any of the MS suite that I won't have a native app for.
PrintNetT print production layout software.
MS Visual Studio for C# development.
Whatever random stuff is thrown at me.
If I need to run a Win app, I can't say, sorry, I'm a Mac person, I can't.
I have to make it work.
On the other hand, in a worst case scenario, I have access to a Citrix box that I can request installs on.
As far as acceptable, what was your CPU and what is your example of an app that could use a bit more speed?