There are huge numbers of missed opportunities with Monster Madness: Grave Danger. For example, even though you get to choose which character you play, there isn't really enough of a difference between each of the four characters to warrant making a choice.
The graphics are okay, but nothing really to write home about. What's really unforgivable though is the sound. While the voice acting is mediocre at best, what really grates is the abysmal audio quality, which is so bad, it makes even Space Taxi sound good. Added to which, there are only a few stock phrases that your character and enemies keep repeating endlessly throughout the game, over and over.
Unfortunately, even with the co-op aspect, the endless tedium of the enemy hordes means that there's not that much to keep you coming back for more. You might play through a couple of levels, but if you actually manage to persevere through all 18 levels, a feat that's going to take you seven or eight hours, you deserve a medal.