Let's not forget the badassity of Metal Slug X.
I don't see anything wrong with integrating some sort of collecting aspect in with it. The Metal Slug games have always had prisoner collecting, but now it keeps track of them along with some other knick knacks. Can't complain about that :)
The bosses aren't too special thus far, but considering how faithful it is to the series in terms of design and considering it's for the GBA, it's really quite impressive and well done. Does a nice job of leaving CT Special Forces 2 and 3 in the dust.