This review will be from an Arena perspective only.
First: Two big issues that absolutely need fixing!
1.) Saving does not work (causing all progress to be lost). I've even tried downloading the game and it still does not always save properly.
2.) The wave does not properly end in the early to mid 30's (happened to me at 33, others are saying 36).
Now, with those two things out of the way, let me say that the gameplay here is by far the best I've ever experienced in a flash game. Totally blew me away with the character customization, weapons, teammates and sheer awesomeness of the bullet system. However, a few suggestions:
1.) Boost the health of the mook enemies in later levels. One shot to the head with any caliber killing them is fine early on but it's laughably easy with high tier weapons like the M249 SAW. Speaking of which, on to the next point...
2.) Rifles (particularly the M249) totally blow all the other weapons away. Its boring in the mid-late levels that the best combo is just two M249's. Just one M249 makes you a one-man army. I did not see the need for teammates when I had this weapon, it makes the game far too easy. Tone down the M249 to 100 rounds and give it a serious range/rate of fire nerf. Maybe even make it slow your movement down.
3.) Other weapons suck ass later. Why bother maxing out any weapon skill besides the Rifle? There's literally no point. Handguns need to be better sidearms. Make the fast switch perk in Dexterity applicable only to Handguns and give them a late/final perk that gives unlimited magazines to make up for their pitiful performance (even at level 30!). SMGs just feel like crappy rifles and need a boost. Shotguns are laughably sad, I don't even know how you'd fix them. They aren't even competitive with handguns.
Don't get me wrong. Fantastically amazing game, but it gets really stale after level 15+ because you just grab an M249 and breeze through the rest of the game. I'd like more options and more balanced weapon playstyles than just Derp Rifle -> Machine Gun -> ??? -> Profit.