I keep the minimum number possible open, since open apps make the phone stutter and slow down. I like reading on the phone, so I usually have fbreader(or more recently mebook) open. I would like to keep email open but it's a resource hog.

I find that having more than 3 apps open will ensure that I cannot take a call. By the time the call widgets show up, the call has been sent to voicemail. Another symptom is that all the sounds don't play properly. Under even medium load, the sounds will start stuttering or get stuck half way (commonly happens on new IM/SMS notifications). The UI will also freeze briefly during animations. Sometimes trying to load the dashboard is impossible if some app is hogging the CPU.

The browser, maps and email are the worst culprits. If I have a browser window open in the background for a day, the phone just grinds to a stop. Nothing short of rebooting will bring it back to life.

Suggestions:

I would like it if the phone app had precedence over all others. Next global UI interactions (notifications, loading the dashboard or desktop, app switching) should have precidence. Those should always work smoothly, no matter how hard any app is working. Finally all apps, widgets etc should have the lowest precedence. This will ensure that one rogue app doesn't bring the entire device to its knees.