* iPod library access (a must have - and many do!)
* Realistic turntable simulation (see Flare Scratch and Tap.DJ)
* Dual stereo audio out (Red Bull BPM has this through streaming to a second device)
* Useful multi-FX and filters with XY pad control (Looptastic has this)
* Access to DLNA/UPnP devices for tracks over WiFi (Plugplayer has this)
* Multiple cue/loop points saved with the song (Tap.DJ again)
* Adjustable crossfader curves (Red Bull BPM)
* EQ plus kills (Red Bull BPM)
* BPM control - syncing etc. (Quite a few, but Red Bull BPM stands out)
* Mix recording (DJay, Tap.DJ)
* Pitch bend (Djay, Red Bull BPM)
I know there's loads of other apps out there, and I know some of those mentioned do other things, but those are the features and apps that immediately came to mind.
As with Guitar FX apps, I am holding out for something near to my "ideal" before splashing out in a bigger way (cash is tight!). I've got Flare, Tap.DJ, Looptastic and Sonosaurus Rex as well as the free version of Red Bull BPM. I haven't tried DJay (no lite version to try) and haven't yet made the jump to the full Red Bull BPM Pro. At the moment Tap.DJ is sucking up most of my time.