The game does offer a dozen different stages; about 30 missions; and eight playable characters, all of whom have an alternate costume you can unlock. There are also skill slots, tons of weapons, and the ability to play online co-op which is fun until your online partner blasts you because you're too far away for them to recognize clearly (the time limits can make this sort of thing really tough to recover from). Capcom has, it seems, tried to flesh out the minigame as much as possible, but the truth is that by its very nature there may not be much of a way to do that – Mercenaries was conceived and executed as a minigame, throwing more levels and stuff to unlock in doesn't make it a full title, just a longer minigame at a full title price.
Okay, we're on the 3DS here, so there are also the obvious questions about mechanics and graphics that have to be answered. The game looks good in 3D; it looks even better if you turn the 3D off, but it still looks good in 3D. The game does succeed in creating that claustrophobic early Resident Evil feel, but I think that it only does that by accident. With the small screen size, in order to have the characters appear of decent size, the view is necessarily restricted.
The controls are all relatively straightforward and work well enough. There is a button available to have you to instantly turn around, but that definitely takes some getting used to – the first dozen (or two dozen or maybe three) that you do it, it is terribly disorienting and you'll find yourself not quite looking exactly where you want to be looking. If only a 90 degree turn is required and you have done the full 180 you've cost yourself time; if a 120 degree turn is required the amount of time it takes you to orient and spin back in the correct direction could be longer than simply spinning there in the first place.