For me its always been the integrated IM into the core. It has saved me from sendinh countles txt messages and calls, and so my dataplan has been pretty much the only fee on my phonebill!

But the best thing about Maemo5 is MicroB. As someone already said, it saves us all from having a gazillion dedicated apps. It has also changed the way I surf the web, as the N900 is now my primary browsing device.