I believe web services benefit from doing less, not more. I believe that allowing the users to stitch web apps together to get increased functionality is better than a web service trying to do everything for everyone. The Facebook app ecosystem is one proof point of this approach.
Evan Williams and Jack Dorsey, founders of Twitter, have talked often of the "constraints" that are built into the Twitter app. You can only post 140 characters in a single message, for example. And because Twitter didn't have desktop client when it launched, a number of them were created and they are probably better than anything Twitter would have created. Same with the iPhone apps like Twinkle and Twitterific.
I think developers of web apps need to think hard about the constraints they are going to apply to their service when they create it. And they need to build an api early on so others can take up where they left off.