Here's a couple of things i thought would be nice in Action!:
Custom codecs
Basically what this means is Action! would support almost any codec beside FICV. Not saying i don't want to it but it adds variety. H.264 and hardware accerelerated support are probably out of the question since h.264 is already in use for quick sync and hardware acceleration is used for exporting. But UT Video and Lagarith might be example codecs you could add support for.
File growth
Got this idea a few days ago while i tried PlayClaw. A counter in the HUD would show me how big the file is getting while i record. For example, recording on 1min would show it passing somewhere at 0,750gb/750mb, 2mins for 1,04gb/1,047mb and so on, depending the settings. This is especially useful on low HDD space, i would know how much space has been taken.
Note that this is just my opinion, i don't know if it's possible to add these.