The Uppers
The sites I enjoyed the most were those which pushed Flash's strengths: blending images, video and sound to deliver highly responsive interactivity. Those bread-and-butter Flash effects like smooth rotations, alpha blends, layers, dynamic lines, and mouse-responsive sounds are still very compelling.
I loved sites with strong graphical punch. I preferred sites which created a more wholistic and rounded experience than is possible jumping between multiple web pages.
I saw quite a few "pan and scan" panoramas - large images which scrolled either horizontally or vertically, with data placed over the image. These panoramas were often very effective for creating a large information landscape, though sometimes the information on that landscape wasn't that compelling.
Content really was a factor when I was making decisions as a judge. The strongest entries were those entries which mixed great Flash effects with non-trivial content.
The Downers
3D in Flash was frequently disappointing. Few places managed to deliver 3D without throwing out usability in the process. Much of it didn't look that great.
I was also disappointed by sites which basically tried to copy what HTML web browsers do best: upright rectangular regions of text with scrollbars, a few images and no sound.
A tip: If you want to use lots of text in Flash, please go to the extra effort to make the mouse wheel and keyboard work well. When page-up/down doesn't work and neither does the scroll wheel, people won't bother reading the text, since scrolling by clicking tiny on-screen buttons gets tiresome fast.
Also, reading a 2" column of text in a small font size is frustrating when you're using a large display. For some reason, designers prefer these tiny fonts and skinny fixed-width columns. I didn't come across a single page with a "make text larger" button, and only a few which let me resize the text flow.
Usability Impressions
I was impressed with the overall level of usability of most of the sites I visited. Many sites had good visual landmarks, minimized mouse clicking, offered lots of reward for interactions, used smooth transitions, supported multiple languages, and gave helpful hints where they were required.
In general, though, most sites were too mouse-heavy - requiring a double-click when a single click or hover would suffice; or forcing the user click on a small dot to start or open enter, instead of launching automatically; or showing a popup panel off the edge of the stage, so the user has to scroll to view it (instead of making the popup automatically center in the window); or animating things so much that you have to chase buttons to click on them. The best sites were those sites which made every click count.
One or two sites were quite complex to figure out - you had to figure out where or how to click to get things started. The worst of these sites came with instructions which you had to read first.
Here's another tip: if you find you have to explain to users how to use a feature on your site, consider redesigning that feature. Time spent reading instructions is time not spent on the site itself. Simpler works better on the web.
Summary
Flash still has that "frontier" feeling - a sense that a lot of experimenting and creativity is part of the community. That's a healthy thing. I really enjoyed my weekend as a judge, although it did go by in a flash.