The basic gameplay is similar to the original Metroid game in many aspects. The player explores a sizable game world, finding upgrades (life, missile packs, and new weapons) and key items as he goes. The gameplay is (barring the bad end halfway through, and the secret best ending) strictly linear, though. Killed enemies may drop health, replacement missiles, or bouncing gold triangles; the latter can be used to level up your weapons up to a maximum level of 3. Weapons gain in power as they gain levels, and many have significantly different behavior at the top level (for example, the level-3 machinegun can be used to hover if aimed downwards). However, taking damage also lowers your weapon experience, so you need to be proficient with the weapons no matter what their levels are.
There's also oodles of secrets in the game, including four hidden weapons and several "bragging rights" rewards. One of which is a timer for speedrunning the secret hidden hell level. Of death.
Just accept you won't be able to perform all the obscure actions needed to get to Hell. A minor hint: the first decision you'll make has an outcome based on the "dramatic" result, rather than the realistic one.
Oh, and for reference, there's a maximum of 50 health and 30 missiles to be found in the game, barring the secret level.
There are several fan-created level editors, and a few people have done some spelunking in the executable, so there's a pretty substantial modding community, even without official support.
Apparently Nintendo liked the look of the game, so it's coming to Wiiware sometime early-mid 2009, with bonus content and optional enhanced graphics.