The Kid Rocks!
First off is the characters. Everyone from Kid Icarus to Mister Eggplant. Second is the sheer mass of level exploration. The levels are incredibly huge and had secrets everywhere. Third I guess is the Nostalgic factor, I remember vividly watching my cousin and brother playing it all the time when I was a little kid and just thinking about it brings a smile to my face. Most importantly is that it's one of those unique games that reflects the time and console of which it was on and as more time passes becomes more than just a game - it becomes a priceless treasure.
Although the characters looked nice, the overall graphics were not up to par even then. Most levels only had a single-colored background with everything else pasted on. Indoor areas in the game suffered as well. Another was some gameplay issues. Flying was erratic in stressful combat and arrow positioning while flying is very hit-or-miss, and that's not to say that when you're on the ground, you're basically a sitting duck.
The Bottom Line
It's a Action/RPG of sorts. You're Kid Icarus, the Son of the Greek goddess of love Aphrodite. A mysterious enemy has unleashed his minions all over Greece, including Mount Olympus. Your task is to track down and kill all of his minions and finally slay the mysterious leader from Mount Olympus before he harness the power of the gods themselves.