The first thing I noticed was that this isn't a bad action platformer. Especially its controls: I had just the day before played the demo of Jasper's Journeys, which was very hard for me to control, so this felt wonderful in comparison. Jumping, walking around, shooting, it reacted instantly and felt right. I think that the main pleasure of a lot of games is just controlling the character, in the Nintendo 64 Zelda games I remember just moving Link around slowly, climbing things, and it felt good. Some games just feel good to control for me, and Space Barnacle is one.
The action elements of the game are largely taken from similar NES games; there's an enemy that circles around blocks from Metroid, an elevator that looks and is controlled much like the elevator in Zelda 2, and so on. It's probably a fairly difficult game but if you got through Cave Story's 'hell' level it shouldn't be too much trouble. It's easy to get lost, though. It's not an extremely short or long game, it took me about a good (and fun) hour to finish it. You may or may not like the ending; it was a but too sudden for me, but I can understand its appeal as well. There may be slowdown if you have an older computer, especially in large rooms or when there's a lot of stuff on the screen at once.
Here's a video of it which will give you an idea of what it's like: