1) Some kind of WAP/simplified phone interface would be really nice. Please don't make it iPhone-only. The normal site works, marginally, on my Palm Pre, but it's agonizingly slow unless I'm on a WiFi connection, and even then it doesn't really render properly given the screen size.
2) The search isn't quite as broken as some people think it is, but it could definitely be improved.
3) It would be nice to work in general on ways to stop getting the same questions over and over and over in some of the forums (like "I'm going to buy my first pair of skis. How long should they be?"). Better search might help. Maybe commissioning some FAQs or stickied threads with links pointing to particularly helpful threads would also help.
4) Maybe some kind of post/thread rating functionality? I've seen this work in other places, though usually in ones with a lower signal-to-noise ratio and less active moderation. Then you can do things like hiding low-rated posts/threads by default, or pushing them down in search results.
Since other people were asking about it: integrated spell-check in the editor works fine in FF3.6.8 for me. If it's broken for you, or doesn't work in other browsers, you should post it as a bug.