Big ones: the whole Google suite. Gmail is clearly the most reliable, but strange things still happen in gmail, like chat dropping, task lists locking up, or crashes. I love gmail, use it all the time, has great features. But I'd love a native client for Gmail even more.
Flickr. Great content and community, but the web interface is clearly clunky. Imagine Flickr on a tablet. Swiping, pinching, tapping. If you had the choice, you'd use the app, not the webpage.
Facebook. Insanely popular, insanely buggy. Try spending 30 minutes going through newsfeeds and photo albums without having the javascript go unresponsive.
Just using those 3 examples, you will see great client front-ends created for those in the next few years. Though maybe not Google if they don't get their head out of the sand. Soon Facebook will do a great tablet client, and it will be a KILLER app.
So what type of professional document creation are you doing in Google Docs?