I've played a great deal of this game over the past couple of days and I have to say that there's actually a lot more to it than it first seems....

1. The first level has three different "styles," depending on which of the bases at the start you blow up: white, orange and red. White has the mountain UFO boss at the end, orange replaces it with the erupting volcanoes from the original Gradius, and red has that AND all the mountains in the level are erupting too, making it much more difficult.
2. There are a good number of secret areas, it turns out, although they're pretty hard to find.
3. Most importantly, loops 2 and 3 (which can be accessed from the start as Hard and Very Hard mode) actually have different level layouts! The basic theme of the stage is the same, but the terrain and enemies have been shifted around. The change on Loop 2 is relatively subtle, but Loop 3 shifts its levels around rather a lot. (It's also incredibly difficult....)