Fred Wilson's 10 Golden Principles of Successful Web Apps (He's at Union Square Ventures and has a blog)  This is a summary of the talk he gave at Future of Web Apps Conference

Speed — it just has to work fast

Instant Utility

Voice — attitude, style, personality, you should feel like your consuming media (CNN, Fox News) when you're interacting

Less Is More — Great services are simple.  Example: delicious (which drives our "what we're reading" section).  You can do very little but still be powerful.

Programmable — APIs, read/write APIs, Absolutely essential to let others contribute functionality to your application.

Personal — Avatars, Backgrounds, UGC, so people can feel ownership

"RESTful" — Everything in the application has a url.  And the url has clean and understandable path.   This way the web can access your app in deep ways.  Build on an open architecture.

Discoverable — Build from the ground up to be discoverable through google, but also through Social Media.  Build from the ground up to be viral.  So it can push itself.

Clean — The app has to be simple visually, not busy on the page.  Lots of space.  Big fonts.  Not too much functionality per page.  Very inviting.  Tumblr is a great example  you just know what to do.

Playful — Have fun.  The ability to play in an application.  Foster a game dynamic.  Like Weight watchers, you establish goals, you get points and report against goals, you meet goals and are rewarded.