Well it's well programmed...
But, the editor itself is not intuitive. There's too many menus, and too many options, and buttons and all that junk. It takes too long to make a level, i just played your pre built ones as examples. I never even finished a level of mine cause i didn't want to spend an hour making a level and figuring out all this stuff and how it works. But I can see that it's well done, so props for that. But, a few little things that can make it better:
When placing items, have it so you can hold the mouse and drag it around to place a line of items like platforms, essentially drawing with the item you're using. That way it doesnt take 2 days to design a level.
On the menu, show all the items you can place down, as buttons. That way it's just open the menu, click the item, place it on the map. It really does help if you can see what your making. Rather than flipping switches around for 3 types of items, i'd rather take up more space and just clutter the screen with a lot of buttons that dont involve arrows. Also, use hotkeys. Nothing says you shouldnt use the arrow keys to scroll, or space to open the menu, etc. It makes it more intuitive.
About the graphics, sounds, etc. Rectangular blocks make a game look clunky (like in the mario game) The sounds you have are kinda monotonous, almost as if the editor would let you compose music yourself. Which would be a neat idea, for even more customizability. Perhaps even draw your own graphics (bitmap data ftw). It's all possible, but it would make the codes larger. You'd probably want to create a compression algorithm for the codes.
Speaking of te codes, I've only seen 2 level editors made in flash where they give you a code (not saying there aren't more than 2, but these are the only 2 i've played). Your game, and my game (see my news post on NG or on my site). Now on yours, i notice that you keep telling people not to post their codes in reviews. Well, why not? Yes, the line breaks and all that junk. So why not just ignore all the characters in the code that involve spaces or line breaks? After a little fiddling, i managed to do it with my editor, cause believe it or not, people WILL post codes in reviews and on forums and stuff. So work your program to the people playing it, dont make your people cater to the limits of the program.
So ya, this is definately well programmed, and defiantely has a lot of effort put into it, and is a good idea. But it's not executed as well as it could be. Plus, the customizability options of it are limited. There's only so much you can do with a character, an enemy, a block, and a coin. You need to add stuff like fake blocks, and blocks that do things if you jump on them or press down when your on top of them, and moving blocks. But i really think that adding doors into it could be helpful. You'd create a few maps, then place doors that link the maps together. You could essentially make a warp pipe in mario with these combinations. Just have some fake blocks, some trigger blocks that you fall through if you press down, and some doors on the pipe.
There's a lot of possibilities for this game that you never added in here. And yes, I know that it will make the codes huge. So who says you have to stick with the codes? Just have like a couple of PHP files and a database on your site, and people get an ID when they make their levels. It could be cool, and then you wouldnt have to worry about codes being the size of this review.
So in summary, you definately need to work on your execution of this game. It's a good idea and well programmed at that, but in a game requiring this much user input you can't afford to sacrifice intuitivity to increase performance or looks. Plus, as it is, there are so many limits to what you can do in the game that it doesn't really hold your attention while playing it. You make one level, then bam, all the levels are so similar it just doesnt feel good. Please play my editor before responding, and I would also like it if you could contace me on AIM, I know your a fan of mine based on the metroid games.