Battleblock Theater is a platformer developed and published by The Behemoth, a developer best known for the born-on-Newgrounds hit Castle Crashers.  Behemoth is a developer that has always had a sort of cheery black comedy sort of vibe going on with its games and that certainly carries on with Battleblock Theater.

That kind of creative art style is what has catapulted Tom Fulp and his studio The Behemoth into their relatively-recent fame after being one of the key founders of Newgrounds back in the early days of the internet, and it's hard not to see how they have gained that fame, and more importantly, held on to it.  Animations are fluid, the character art is inventive with a variety of different characters available (including a Castle Crashers character for those who own both games), and it all gels together with the narration, sound design, and game mechanics very well.  It is a very highly-polished game.

Mechanics are varied and tight,
and the game introduces them at a good pace

There are basically two main mechanics at play other than the usual simple mechanics of combat platforming: the actual combat items, and the different blocks which can have different effects.  Some blocks will vault you about, others will kill you with electric shocks or drowning in water, yet others will slow you down or allow you to grip to a vertical surface.  They're introduced at a steady and well-paced clip, and the levels as designed make very good use of these blocks and have some particularly devious ways of both needing to combine various blocks to get around to the objective gems you have to collect to be able to exit the level, and in finding ways to combine those blocks to kill the player in pretty creatively insidious ways.

The weapons are fairly varied as well, though I will refrain from commenting on specific ones since most of them come some ways in and as a result talking about specifics could be considered spoilers.  Nonetheless, each one has it's own specific way of functioning and they offer all kinds of interesting possibilities for the cooperative or competitive multiplayer above all.

Gameplay is smooth and responsive
on both a keyboard and a controller.

One of the biggest things with these kinds of platformers which is why I generally avoid them is because they're usually quite terrible on the keyboard and as some of you may know, I have pretty terrible arthritis that doesn't agree well with the XBox controller.  BattleBlock Theater however, while recommending the controller as the best way to control the game, is perfectly responsive and playable on a keyboard, and I was able to get through the game without any problem with the keyboard controls.  No doubt it's probably better to use the controller in competitive multiplayer, those keyboard controls are perfectly fine, quite good actually.

There were a few problems I had with the game, but most of them were minor.  The Steamworks Market integration doesn't tell you what the going price for items are nor provides a cost history so you can possibly get ripped off if you're not researching your purchases or sales of game items to begin with - do be careful if you're using wallet credit for that as there is definitely a criminal element on Steam that will take all kind of advantage if they find you and consider you a mark.  The game plays in a window that never quite seems to go fullscreen for me, and occasionally the mouse escaped the window, and the music played when still alt-tabbed.  A few of the King of the Hill maps make it really difficult to dislodge a team that is currently on that capture point.  But all of these are minor quibbles that don't detract from the load of fun I had with the game.