Maybe I'm just in a bad mood, but with all these wonderful "open standards" RSS, SOAP, Atom, XML, etc, etc, why can't web apps work better together?
For example, here are a few of the web-based apps that I use regularly:
I do consider these "apps" even though you might also call them "services", but anyway. Most of the stuff listed has some sort of API, but none, natively, work together. With "traditional" desktop apps, the OS provides some sort of interoperability, whether it be as simple as file-format compatibility or using a clipboard type of tool. But on the web, there is little that these apps have in common, other than my data. So while its great that most provide an API, unless I feel like writing the code to tie it together, these apps have little data sharing.
It is even worse if you throw in some desktop apps, such as Picasa or Onfolio. Granted, they come from two different companies, but why can't I easily publish to Flickr (or any other non-Google-photo service!) from Picasa? How about calendaring - why can't a single device sync with both my personal and work calendars reliably?
While having the capability for interaction, unless quality 3rd party software or (gasp!) integrated interoperability becomes the norm.