First off, i love aTV Flash. I tried XBMC before but it felt too much like a PC, so it's great to get all the benefits of jailbreaking while still keeping Apple GUI.
The only feature that i miss in aTV flash is audio delay compensation. I'm having lip sync issues between my TV and my receiver, and it would be great if i could just set the audio to be delayed a number of milliseconds. VLC has this feature, and i'm pretty sure XBMC has it too. Out of sync audio is really annoying so it would be great if you could include this fairly simple feature.