Jan 19, 2012
The first time I rooted and unlocked my bootloader, then flashing AOKP 4.0.3, all my apps started to be downloaded automatically and installed after signing in to my google account. I've just flashed AOKP 4.0.4 (after wiping data) this time, but my apps did not install automatically. In the Play Store app, all my previous apps did show up in the "My Apps">"All" tab, but I now have to go and install each and everyone manually. Any ideas?
First, try rebooting - that usually fixes things.

After flashing AOKP, did you flash GApps? This is usually the main problem when you flash a ROM and you forget to flash GApps.
Must have flashed Gapps or there'd be no Play store.

I've heard of this happening a few times on various forums. I think it's something to do with the apk name of the store being changed in the new version to use ICS naming conventions - vending.apk has become phonesky.apk. One solution is to flash an older version of Gapps and then update them in the play store.

Never noticed it myself as I flash Titanium backup when I reflash a rom. I then stop the initial setup (tap on all 4 screen corners) and then restore everything through TiB including app data. Only once I've done that do I log in and sync everything.

A much easier and quicker solution.
Yes, I can second what GrenW says and yes, Titanium Backup is probably the best solution. However, in your case you have to do the chores manually and in my expoerience the easiest way is to go to your PC and log into your google account and then go to the Play Store > My Android Apps > install from there and they will be sent to your device in very quick succession. Once you have everything back in place get yourselve Titanium Backup.