The difficulty curve is smooth and the game is fun. It'd be great if there were more levels, though.
I was thinking about leaving it as-is, but I think I would rather create more levels - starting games is the hard part for me, not finishing them, so I may as well make the best of it - so you will probably get your wish. I actually wanted to do one with a stained glass window, but I didn't want people to construe this as anti-religious or something. What do y'all think? Also, if I add more levels, should they be interspersed with the existing ones, or tacked on to the end? Basically, should the game get much harder than it already does?
Right now the scoring system is linear, you don't get a big gap between novice play and mastery. On the other hand, Window Hater is exactly the kind of game that begs for a combo-based scoring system. Finding ways to incorporate combos into my play strategy would be fun.
In short: add an in-game score counter, base score on combos as well as time left.
I was thinking of doing this already; the game is well-adjusted for it. I know pretty much exactly how it'd work, too, so it shouldn't be too hard to implement. So a definite yes to this.
Because the main character and smoke are depicted in the same color, it can become difficult to distinguish where exactly the character is. This disorienting effect is exacerbated by those juicy explosions shaking the screen.
In short: change the main character sprite to a more stand-out color.
I meant to fix this in the last update, but forgot about it. You are far from the only person to make this comment.

The gas helicopter in later levels lends itself to a deeper strategy: once it deploys the gas, set it on fire to heat up all the nearby windows. Of course, you also run the risk of setting yourself on fire.
That is a very good idea, but I don't know if I want to do it because I think the player is inclined to hold down the fire button pretty much the whole time, and so if they were hit by the gas it'd be an automatic death sentence. I suppose they could adjust to it, though, and it is a pretty cool idea. I will think about it. I won't be able to work on this for a bit anyway, because of computer-related issues.
Personally, I would like a little more of a buffer for falling off of the stage. My first couple plays, I died while trying to get the low windows on the second stage. Also, it would be nice if there were a few window-shatter noises, that played randomly when you broke a window. It got kind of repetitive. Maybe just a pitch modulation of the noise based on the window size..
To your first comment, yeah, I feel, myself, that it is somewhat unforgiving in that regard, so I'll add a buffer zone of a few pixels. And I tried having multiple window shatter noises (pitch modulated) that play randomly, but I did not like the way it sounded; I found it distracting. There are already three different shatter sounds, though, for different sizes of windows.
@ Adam94 - thank you; your enthusiasm is heartening.
@ Theta - if you think it's hard now, you should have seen the first beta release.

It's a hard game, yeah. I'm always pretty good at my own games, so I'm not sure quite how hard they are. I think I've adjusted this one to where I want it to be, more or less, though.
@ moi - yes, it reminds me of that, too.
@ bluej774 - the screenshots are older. I reversed those levels because I thought that the one with the wall of windows was only really very exciting when it had the blue helicopter, but I wanted to introduce said blue helicopter one level later for a smoother curve. So yeah. I'm glad to hear it's fun, though, and especially that you liked blowing up a certain thing, because the heart of the game is supposed to be that distinct gaming pleasure of destroying windows.
On this note, are the explosions visceral enough? Do they provide the player with basically a drug that will keep them playing despite the game's difficulty, just so they can see/hear/feel it one more time (and another after that, and another after that, etc.)? The whole game really revolves around that, and so I want to know if it's effective, and, if not, what the heck else I could do to make it so. I mean, it's a game about destroying stuff you shouldn't be destroying... I want to milk the guilty pleasure to its greatest height.