Contra meets Metal Slug meets Worms.
Overall it is a side-scroller, with great movements as in a platformer (on a keyboard I recommending binding Jump to the same key as Up), a button for melee attack, a button for special attack (airstrike, slow-motion, and so on), and a button for the main attack which is usually shooting in the direction the character is facing. I really like that the movements are very fluid and allow for a great variety of moves such as wall-jumping or grabbing ledges or sliding down cords.
The game is both action-packed and violent, yet subtle. The idea to switch between famous action characters with different abilities is great as it creates a lot of variety in the situations. Some characters are melee only, some characters are more suitable for picking off enemies one by one, some characters blow everything up (the levels can be destroyed, and the level design is really well thought in order to give the player solutions to progress). Each character can influence how the player approaches the next place to assault.
The levels and enemies lack some of that huge variety that the characters have, but as the updates progress there is more and more, allowing for new situations to occur and renew the fun. Although, even as it stands, the game is already very very good and well worth the investment. Easy pick-up fun that I come back to quite often. Already one of my favorite games ever.
There is a level editor, so the community will keep the game alive and with new content for a while.
It is possible to play Coop in offline (same screen) or online (might require to open ports) multiplayer. Really adds a LOT to the fun of the game. I think that two players is the sweet spot to play seriously, as if there are more players it becomes very chaotic and the levels get destroyed quickly, sometimes hindering progression.
My only complaint is that some of the early characters are dull and unoriginal in comparison to the gameplay variety and cool specials that were added later.
If you are not sure about Broforce, try out the Expandabros on Steam.