Higher expectations are the major issue: undo being the canonical example. The general problem of distributed system data synchronization is another: things tend to demo well enough on a local development instances, and then fall to pieces in the unreliable field. Showing synchronization status, merging changes in a semantically correct way, useful UX affordances around all this, etc. is all very difficult work.

Of course it all depends on the domain and architecture of the app in question, so counter-examples and exceptions are easy enough to find.