Thanks, everyone!
I wasn't counting, but I figure that I probably spent about 12-14 hours on the game each of the first 4 days, then about 4 hours on each of the last 2. I tried to do as little erasing and correcting in my work as possible. Nearly all of the riffs and melodies and beats in the music were what I first composed then didn't spend time tampering with, same idea with the graphics.
On the difficulty:
The type of game that I find fun is one in which you're constantly learning, as opposed to something that only tests reaction time or other latent skills and abilities. The idea I tried to implement is that the player will fail a lot, but can beat every hard part using perseverance, by adjusting their strategies and remembering to correct whatever they did that went wrong the last times. Don't expect to get by on your first try on every level using skill alone, the game is not intended to be played without failing (a lot).
That, and also the fact that I enjoy hard games so I made this game hard. :p
@Jon
Yeah, Mega Man was definitely the single biggest influence. The idea was basically to make a MegaMan-like game with my own ideas.
@triptych
I'll have to consider putting in some kind of a story before it's done. Maybe I can have story-related boss fights after you've beaten certain numbers of levels, I remember I liked those that were in a few Megaman X games I've played and MegaMan 7, I think.
@Alexin
I've done nothing fancy with the camera movement, so if there's an issue with it, it's not my fault. :p
@Legendary Hoamaru
Hey, I see someone beat the game!

The missile barrage was sometimes choppy for me, too. I kept it in to see if anyone else would notice, so now I know I probably should probably keep the maximum number of stuff happening at once below that amount for the final game.
Funny that you did Nightmare first, it was intended to be the hardest level (aside from the final).
Here's what I thought the order from easiest to hardest would be (and probably is for myself):
Suburb, Water, Flying Ship, Digital, Nightmare, and (obviously at the end) Final
And about the double tapping, do you mean with just dashing or with charging too? I made the dashing needing relatively quicker tapping, because I would keep accidentally dashing into holes when I was trying to walk. I can easily make the charging tapping more lenient and I don't think anything will be lost by doing so.
@Furby
Output a swf, open it, "File," "Create Projector"
@almyki
There are checkpoints! Of the first 5 levels there's one in the middle and one right before the boss. If there's a room devoid of enemies, then it's probably a checkpoint room. Although a game over means you go back to the start regardless.
Yeah, I did all of the music, graphics, etc., except for some help with bugs from Greg and Legendary Hoamaru.
If I get frustrated with one world, I can try another until I get frustrated with that, and I may eventually go back to chip further my way in that old world later.
Haha, well said! Glad to hear that because that's what I was going for!