The collision detection is off. The bouncing feels awkward. The second level is too frustrating.

Hi, neat game. I think it could be very good, if you get rid of few bugs and reconsider some things.
What I noticed:

- Mostly the graphics. It would really make your game look more professional if you spent some time with drawing the main character, the tiles, and so on. Now it looks just as Microsoft Paint. I don't say it couldn't work, but you should at least ensure (for example) that a wall is straight (not randomly placed tiles over each other with holes and gaps), black lines on wood are narrow, etc.
I don't know which drawing software you used, but Flash/Photoshop/Gimp/Inkscape/.../ might be better, as every model would definitely seem more fluent (especially in Flash, because it has vector graphics).
There are even drawing tools which can be opened online (http://www.ratemydrawings.com/startdrawing.php).

- Sometimes, the main character floats in air, although the ground is much lower. I don't know how you detect collisions, but if you make whole tiles impassable, you can't have diagonal slopes there, because we will have to go over invisible stairs no matter what. Either change the collision detection, or put 'stairs' everywhere (e.g. the way how it is done in Mario).
This link might help you, but I don't how well you can read in English (http://www.hobbygamedev.com/adv/2d-platformer-advanced-collision-detection/).
Yeah, and it is impossible to move while jumping, if you were standing by a wall before. So you always have to move a little backwards to be able to jump over a wall or so.
Also, I wouldn't recommend 'Shift' as the jumping key, because it pops up the annoying Sticky Keys (Klawisze trwałe (:D)) pop-up after five presses in Windows XP. If you don't like up-arrow key, the Spacebar is the usual pick for jumping. It's also bigger than the shift key, so it helps, because in platformer games like this, it needs to be pressed quite often.

- Please, make the character move faster in water. I liked that there's is an animation of falling in the water (just adjust the timing, because it plays in front of the main character, not above him), even the spikes sparkle, that's all good, but no matter how much I press the arrow keys, I can barely move left and right, that means I will almost always touch a spike.

- Wouldn't it be better if the spike hurt you just if you touch its tip, not its side? A clearer health bar might be better than the green/red cross, because players might get confused until they get its meaning.

- Touching a ceiling makes you stop moving forward. So you are jumping over a spike, touch the ceiling a bit, thus you stop moving forward and fall down on the spike.

To be honest, because of the last two notes, I couldn't complete the second level (I only got to the part with the lever), so I can't judge the content following it, sorry.
You might say it's a matter of difficulty, but there's a difference between hard difficulty, which is generally fine and challenging, and unexpected invisible walls with slow movement speed, which might be challenging too, but more likely it will annoy the players.

To sum it up - I think it will be a great game, I just recommend redrawing few models and adjusting the collision detection.
I hope I didn't make you upset by this review, I just posted some tips which I believe could make your game better.
Good Luck. ;)

For an alpha version already good, so keep working and i'm looking forward to see the final version.

Nice game, of course you had to include a water level early in the game! Improve the graphics a bit (I know it's in alpha) and you'll get a better score from me.

wspaniały!