I'd say WebOS's weaknesses are 100% hardware. App store, sure, but that's because no one has a Pre or Pixi because they are cheap pieces of plastic crap. Put WebOS on the HTC Supersonic and just try to keep that thing in stock.
The default music playing app is much better on the iPhone. Other than that you can get videos from Youtube and break.com. You can play music in the background with Pandora, Grooveshark, Slacker Radio, or Radio Time. You can upload photos and video to Youtube, Facebook, Flickr.
As for a flexibility, look at the WebOS hacking community. WebOS is running on a Linux that isn't very far away from stock (closer than Android for sure). Just last week they got the QT framework running on it.
