The iPad Mail application lacks a *lot* that you might be used to if you use the Mac as your everyday machines.
Different signatures is one thing you'll miss.
Personally, the biggest thing that I miss is the inoperability of Smart Folders. They're 100% with my MacBook and iMac, but fail on the iPhone and iPad.
We don't create these folders because we do *NOT* need them!!
It's possible that iCloud interoperability between OSX and iOS will solve many of these issues, however. We'll find out shortly.