I made a one level Commander Keen ripoff in Java in high school.
Key features:
- A stick figure protagonist whose legs moved when you were walking but not when you weren't (hey, the original version just played an animated .gif file all the time, so that's something). He even faced the right direction.
- Enemies that took shits (like in Commander Keen) that would kill you if you touched them
- The enemies would duck under your shots with a chance influenced by the difficulty level you chose
- Awesome art created in Flash by someone who actually knew what they were doing (i.e. not me, I just did the stick figure man), including animations for enemies dying, moving, and ducking (and subsequently sticking their tongue out at you)
- Awesome sounds - the classic Mario jumping sound, a desert eagle gunshot when you fired your gun, Monty Python and the Holy Grail's "And there was much rejoicing!" when you won the level, Die Hard's "Yipee Ki Yay!" when you obtained an item, and plenty more
- A grenade you could pick up
- Destructible environments! (well, you could blow up a tower with the grenade)
- High scores (highest # of enemies killed and fastest time were recorded)