I guess I have to give Zaxxon originality points for being the first isometric shooter (possibly the first game to employ an isometric perspective?), but honestly the gameplay was never all that great to begin with, and it certainly hasn't aged well.
You control a space ship doing a series of endless Star Wars-like trench runs on some futuristic defenses such as laser turrets, homing missiles and other spacecraft. I guess we have to presume the ship we control is benevolent and these defenses aren't like the last line of resistance against a power crazed empire or something. Anyway, the game is unique in that elevation plays a major role. Shorty Zaxxon gotta get low to hit a lot of the turrets, and also to blow up fuel canisters that somehow magically refuel you when they explode. There's also some giant douche of a homing missile that always seems to be after you and you have to shuck and jive up and down a bit to shake it off.
The game was popular in the '80s and ended up spawning a bunch of ports and remakes, but I really think it was the uniqueness of the concept and the colorful and detailed (for the time) graphics more than the game actually being a lot of fun to play. It's slow-paced and pretty clumsy, and hard to get the hang of since judging elevation accurately is tough.
* Gameplay Video