From the top of my head here are some improvements Apple can't afford to forget on the iPhone OS 4.

• Allow a finite number of selected third party app types to run in the background. This includes streaming audio apps (ie: Last.fm, Spotify,...), GPS apps (ie: vocal guiding, speed camera alert, track recording...) or instant messengers and SIP clients (ie: Skype, iSip...). And to a lesser extend, Twitter clients to get reliable alerts on mentions or direct messages.
• Include a 2010 class notification system, as in get rid of modal message boxes. Email notifications must have both the sender name and subject.
• Use the lock screen to display more than the date, time and slide to unlock. Hints: appointments, notifications, birthdays, RSS feeds,...
• Be smarter with location and time aware settings. I manually put my iPhone in airplane mode every night because the push notifications are not aware that I do sleep at night.
• OTA OTA OTA - as in over the air so most of the everyday stuff can be done without the PITA iTunes (podcast automatic synch, firmware update,...).

There is no need to have two games running in the background fighting over 3D ressources. It's also pointless to have two audio apps mixing their audio into an unusable mess.