-Add a background, pure black is boring and can come off as lazy. A simple background will do. Maybe something from VVVVVV?
-Blocks could look a bit better. I recommend making an outline of the solid wall, instead of 32x32 squares everywhere. (here's an example: https://i.imgur.com/5qrM1xS.png)
Minimal graphics = lazy? Really? Honestly SpaceH3R0, I think the graphics look fine and I don't think you need a background for this area. You're channeling are real ZX Spectrum look and it's working out. The style you have is reminiscent of VVVVVV but doesn't copy it which is really nice. I also don't think the walls need lines.
-Maybe make the color of the spikes change to match the blocks?
I personally think this would make the game go from a nicely minimal to boring, so idk why this has been suggested. The spikes should pop out, not blend in. Though maybe SpaceH3R0 you could make the spikes the opposite colour to the blocks in every room, that might be a neat little graphical flourish.
-After you get the secret item, make it take you back to where you were instead of the beginning.
-Maybe make the flip lines play a sound effect when you touch them?
Agree with both of these, as long as the flip sound isn't annoying.
I thought this was okay, but you're about 4 years late on the whole VVVVVV fangame business so you're gonna have to really up the quality in the next few worlds to make this standout.
My points of criticism would be:
-The difficulty curve doesn't make much sense. None of the game is hard. But the second save is noticeably harder than the first and then the third is noticeably easier and the fourth noticeably harder. The overall difficulty is fine, but inside of the demo it's a bit all over the place.
-There are some pretty distasteful design choices you make: Offscreen jump into a drop? Come on. It looks like a set-piece ripped straight out of the megaman room in the engine (in fact I'm pretty sure it is, since it's in the same position as the drop in the engine) and it contributes to the uneasy difficulty.
-On the second screen you could get stuck where you weren't meant to by using the flip and then going back into the first room. I thought this was part of the secret and was then disappointed to find out how easy it was to spot the secret entrance. I would consider moving it there as it takes a bit more thinking to get to. Otherwise make sure to seal your gaps so that you can't get stuck.
-I don't understand why some gravity flippers were the VVVVVV lines and others were the stock engine arrows? Choose one or the other, don't mix them. It looks really messy especially when you can land between the arrows and give yourself a breather which wouldn't be allowed with the lines, in keeping with the pace of the level.
Listen, this wasn't a bad effort, but it was also really untidy. The main problem for me is, there's only about 5 rooms, not counting the secret. It took me less than 10 mins to beat. Why release something so unfinished, and then respond to criticism by saying "This is the alpha version." or " I know, but I don't have X yet."? No one really wants to download and play something so small anyway, let alone when even what's on show is unfinished. SpaceH3R0, finish a few areas first and then release (or just finish the whole thing, what's stopping you?) and I guarantee you'll get more buzz around this. If you put it on the wiki when it's finished it'll definitely get played by memebers of the wannabees etc. But right now this isn't worth playing for anybody because it isn't even one whole finished, polished, area.