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.