Jamestown has one of the very best soundtracks in an indie game (and I play a lot of indie games). It would fit in a huge budget movie brilliantly, gives the game a sense of urgency and scope even though it's just a vertical shooter. Listen to it: http://finalformgames.bandcamp.com/album/jamestown-soundtrack
Then there's graphics. I'm a sucker for good pixel art and Jamestown really delivers in that department. The theme is pretty insane (American colonial wars on Mars) but again, this was a given for games made 20 years ago. I'm really happy there are still devs who writing that kind of silly but fun story nowadays.
Game mechanics are both simple and skill-rewarding. You get a Vaunt meter that fills up when you collect gold from destroyed enemies, and when it's full you can activate the special. You'll get a shield for few seconds and your weapon will turn a different color and do more damage. If you want to sustain this you'll have to keep getting more gold. Larger enemies drop bigger amounts but you can also do combos on some enemies - destroying parts of them in a specific order or hitting them in a particular moment. This adds so much depth to the gameplay.
There are only five levels in the whole game but each one is meticulously drawn and designed. The devs managed to convert this limitation in their favor. There are six difficulties to choose from and each new level cuts out the easiest one - that way you'll have to play the last level on one of the two hardest difficulties - Divine and Judgement. And believe me - this will test you. It's not as brutal as Japanese shmups but it gets intense. There are also 4 ships to choose from with different weapons and specials (the Gunner being my favorite). The DLC adds 4 more ships. Small number of levels didn't stop the devs from making a big number of shorter, bonus ones. You unlock these with coins you earn after your level evaluation. There's also a gauntlet mode where you play levels in order. Tough as nails but really satisfying.
There's local co-op and it's done brilliantly. You can fully customize the controls and play 4-person co-op with one gamepad, a mouse and a keyboard. There's even an achievement for completing a level that way. No online though, not a problem for me but some people can see it as a serious flaw.
I simply love this game. Wish more people would play it. It really reminds me of my Amiga days.