This version of Mario Bros. is pretty decent. Sure, there are obvious compromises, but that's to be expected when an arcade game is brought home to the Atari 2600. All of the enemy critters are here, and Mario and Luigi are free to cooperate or compete as their players see fit. The graphics and sound won't shock and awe anyone, but players can still easily tell what game they're playing. Really, this isn't bad at all, and yet, something feels missing.
2600 Dig Dug, released the same year as 2600 Mario Bros., proved the console is more than capable of juggling multiple independent enemies, every one of them able to go virtually anywhere on screen. This was done without any sacrifice to other game mechanics, and was even accomplished without too much of the 2600's infamous flicker. The following year, Stargate and Crystal Castles demonstrated these capabilities even more. Compared to those games, Mario Bros. feels very constrained. Only one enemy at a time can appear on a given platform. If an enemy is at the edge of a platform and would fall onto a platform already occupied, it will instead reverse course. There is no logical or "in character" reason for this behavior, just an artificial barrier to ensure the game program doesn't have to deal with more than one enemy per platform. This obvious and easily exploited limitation ultimately makes 2600 Mario Bros. less satisfying, especially when you know the console can do so much more.
Maybe preserving the simultaneous two-player action of Mario Bros. required enforcing more limits in enemy movement. Joust, the only other simultaneous two-player 2600 game released by Atari in the same year as Mario Bros., also has very obvious limits on how enemies can move. However, there is also the fact that Dig Dug, Stargate and Crystal Castles all benefit from extra memory housed inside each game cartridge, allowing them to better track things like enemy positions and movements. Joust and Mario Bros. were not given the luxury of extra memory. In Joust, at least, the changes and constraints actually work together to create a game that still succeeds in being just about as fun as the original, despite all the differences. Mario Bros. isn't as lucky.
Again, this port isn't bad, and will be fun for any Mario fan willing to live within its constraints. It's just hard not to wish those constraints didn't exist.
Grade: B-.