Ignoring elitist video quality, right now, Action! is the best all round screen capture software on the market with: low resource usage, low file sizes, ease of use, seperated* audio, and playback recording. I might have missed out features but those are the really key ones.
Im my opinion though what stops it being the best is a few things i'll now go over each, and how it could be improved to make it stupidly good.
Separated audio - Right now the ability to separate game sounds and microphone is fantastic. Action is one of a few that have this ability but is it still put in the dark compared to DxTory. and what stops the professional using Action is the fact 2 channels is not enough. You have the Game, microphone, Voip with friends(multiple cases sometimes), music. So being limited to just 2 is just not enough. Making the ability to select numerous sources would take the software to the next level.
Video Quality (KBPS) - In all honesty, Action is average. from my usage im guessing Action on high records around 30kbps max. when the likes of Shadowplay goes up to 60k and DxTory even higher. ofc you may be limited by the cosex you use and the ammount of write speed you want to output, but the ability to go higher would be nice.
Ingame Overlay Inprovements. - right now its horrid, what we want to see is, ingame fps, recording fps, recording state(on/off/timeshift/microphone)
These are just my thoughts and opinion on the brilliant software you guys are crafting right now. i use it everyday, and would love to see it improve, because right now its at the stage, it will do. and if i had an ideal setup, i wouldn't even consider Action because of the lack of necessary features right now.