First off: love this game. Getting alright at it now, completed the city of gold run, unlocked all hidden doors etc. Favourite playstyle is speedruns, fastest time is 7mins43, or 9mins 54 without using jetpack.
A few bugs I haven't seen mentioned:
1) Sometimes when bats kill you, rather than flying to the ceiling they go straight into their "hanging" state and slide along the floor. (I think it may be to do with being in confined areas)
2) Spikes that are hidden directly below a wall tile can kill you weirdly if you jump at them from the side.
http://img21.imageshack.us/my.php?image=88940051.png3) Spikes don't kill you if you jump on them from up to 2 tiles high, but if you bounce (e.g. after getting hurt from a fall) even 1 pixel down onto spikes you will die. (Same goes for the princess - she will always die on spikes because she is always in "tumbling/bouncing" state rather than "falling after jumping".)
4) There is no way to quit the game from the options screen. Esc, the X button etc all load to Spelunky. There should be a quit button.
5)If the giant spider spawns with only 1 tile above him (so some of his underside is exposed), and you jump down onto him while he is still clinging to the ceiling sometimes you can pass right through him. You will arrive at the floor beneath him without him jumping on you, but as soon as you move once grounded he will fall on you. This could be remedied without changing collision behaviour if you only let him spawn with his full body attached to the ceiling.
Opinions:
1) I don't like dark areas. They don't really add anything to the game in my opinion, and seem more like a programmer playing around with GM lighting. Personally I think you should remove them, but if not, I recommend forcing them to occur in every world, or a fixed number of times per game. It's unfair that some runs you may not have any, and in others you'll have all 4. I know this game is about randomisation, but when you're doing speedruns a dark level always adds 30-60 seconds to your time, and it doesn't seem fair that your time should be based so much on a diceroll. The other random elements of the game can be overcome with enough skill but this cannot. You simply can't run at full speed with only the halo of light to see by, arrow traps will kill you before you see them. Either 1 per world, or none!
2) This may seem picky but I also dislike the exit section that looks like this:
http://img245.imageshack.us/my.php?image=67697273.pngIt's really annoying trying to jump up that little hole, and it's such a common exit. You should fill in that botton gap and make it look like this:
http://img179.imageshack.us/my.php?image=46197257.pngimo.
3) There should be more cumulative stats! Besides number of wins and deaths (the latter of which I have accumulated an embarrassing number), you could have all-time kills, saves, money accumulated. Those things would be good to see on the score sheet.
4) There is currently nothing to indicate you have completed the city of gold run besides a high gold score. You should get a different ending, or at least something on the score sheet to say you've done it.
Also please fix the falling bug where you get hurt after falling a tiny distance, that is really annoying!
Many thanks, (sorry for essay)
Rain.