One big difference is that, because IOError handling is handled separately from HugsExceptions, we can't readily implement

throw :: Exception -> a

But we can provide

throwIO :: Exception -> IO a

(which I'll try to persuade the GHC people to ask to their implementation).

This is called 'ioError' in GHC, because the Exception type is now a synonym for IOError (in GHC 5.00). This makes it easier to port code that uses IOErrors to use Exceptions - the Exception interface is just an extension of the existing IOError interface. Cheers, Simon