I really don't need, and don't think most people really need, on-access scanning. Setting-up a download manager to run clamwin, as well as scheduling a weekly full-system scan, is plenty of protection.
What I'd say everyone needs is for ClamWin to perform much better than it does. I know it's not easy to port apps from Unix to Windows, but performance really should be a high priority. Right now ClamScan takes more than 4X longer than Grisoft AVG to scan my system. That's such a huge difference that I can't really recommend ClamWin as of yet, even if it was full-featured, and had a nice, on-demand scanner. The memory footprint could use some improvements as well... 18MBs for ClamTray, 22MB more for ClamWin, and 30MBs more for ClamScan.
On that note, a simple user-executed program that could update the virus database (via Task Scheduler) would eliminate my need for ClamTray all together. I know I can download the daily.cvd manually, but putting together an app which checks local and remote modification times isn't exactly trivial, so any simple methods would needlessly waste bandwidth. I've tried invoking FreshClam directly but it errors out, looking for .\clamd.conf, and it's operating is a bit too cryptic for me to guess what options ClamWin is passing to it, or what options it uses its config file to be in, without an example.
Fast start-up of the scanner would also help matters. It seems like the ClamWin window sits idle for about 2 minutes before it starts actually scanning even the tiniest of files (not that my computer is top-of-the-line). Also, on this note, it would be a big improvement in both performance, user-friendliness, and keeping the memory footprint down if ClamScan would detect if another instance is running, and simply pass it the filename to scan when it has finished with the current one. It's quite a mess when I'm downloading a dozen tiny files, and a dozen instances of ClamWin pop-up almost at the same time, max out my CPU, and still just sit around doing nothing for several minutes.
I hope some of these performance improvements can be made. Windows could really use a good open source antivirus scanner.