Reminds me a lot of my old animations and tests years ago, unfortunately I lost most of them when my HDD crashed. Not bad man, you need to ease more though.
There are a total of 3 frames before the punch. This is bad because there's very little anticipation and build up for his epic fucking slam dunk punch to the chin that you want to see. To fix this, slowly ease up from "fighting stance" to "punching position". The punching position would be just when he's about to punch. Then when you're there, BLAMO! Make it go from totally about to knock red guy's lights out, to fist in his fucking face to really add some heat to this punch. Just start off slow, then a little bit further away from onion skin, and then right when it's in position to punch, slow it down a little cause he's basically done "charging up" as you could say.
More frames inbetween actions will show greater fluid movements, if done correctly of course. Because there's more frames inbetween actions to show us how smoothly things transition making it look all sexy to the eyes. Just a few frames will leave us wondering, "What the hell happened I couldn't tell!!", cause there's no transition time. Now you can't just add frames inbetween shit and expect it to look marvelous, you need to transition your shits, and get good poses, which will come with due time so long as you practice practice practice.
Here's what you should do to fix your combo. Have him actually lean backward (which you did do a little :D) before the punch, and have his arm charge backwards like a slinky before he releases it. Then fucking blamo, send that shit flying forward to annihilate his opponent. You can exaggerate this to really get some power going if you want. Here's an excerpt from an old animation I did that displayed this movement a bit: http://imgur.com/a/TMOot (Warning, this is just a rough example, not at all the perfect example of this.)
If you want more of a slight jab, just do what you did but carry the movement more and really show it in the spine. The spine is kind of stiff the entire time, but whenever you throw a real punch, you'll always move your back forward a bit. And you have a good opportunity to make this flow like water, cause if you lean him forward as he punches, he walks forward anyways in your animations to do his next attack! This is going to show clear inertia and make it look much better. Remember, an object in motion stays in motion! In your anim he kinda stops for two frames, ruining the inertia of a punch.
You have foot placement down, and all you really need to fix is your easing and transitioning of actions to really make your animations shine. Sorry for the long advice, but hopefully it was helpful to you, and I was in the mood to help someone out. Mods feel free to correct me on any bad advice I may have given, as I am not a pro.
Chris: wow man (O_O
