This game might have been okay, if it weren't for one fundamental flaw.
However, this game has a MAJOR flaw that Contra didn't: Contra gives you a limited number of lives; once you run out of lives, you have to start all over.
Metal Slug, however, does give you a finite amount of lives, but this limit means nothing. At the old arcade, you had to put in more quarters if you ran out of lives; a ten-second countdown would begin once you ran out, so you had to make up your mind about whether or not to spend the extra coin quick.
Metal Slug Anthology, however, doesn't require the extra coin, but nor does it make you start all over. If you run out of lives mid-battle, just press the + button and you're right back in the thick of things, with three "lives" to boot. By exploiting this, I was able to pass through all of the Metal Slug games entirely in less than three hours.
So, this game gets a 2.5 for the same reason that Tetris Worlds on the GBA got a 2.1 from Gamespot: It breaks a should-be classic by making it way too easy.