Hi
Cattus and welcome to the WYMeditor forum!
I think that the main difference between WYMeditor and other "classical" WYSIWYG editors lies in philosophies behind them.
Most of the WYSIWYG editors available today attempts replicate the functionality of desktop word processors, and since most people know how to use a word processor that might not sound to bad. The problem with these editors are that they're way to powerful - they present the user with so many functions that most people just get confused. And since most of these editor also comes with "all batteries included", which of course might be good if you want to get thing up and running quickly, they're rather heavy weight. Most of these editors
do generate valid (x)html, but since the content (most of the time) lacks any semantic value that doesn't really matter. If you know what you're doing this might not be a problem - but judging of what I've seen, most clients don't.
With WYMeditor we've taken another approach, trying to give the content meaning by emphasizing the document structure (which you mentioned). Things like the size, face, colour, alignment or weight of the content doesn't belong in the editor at all - the presentation should be taken care of in the stylesheet. By not including (or disabling) most presentational features WYMeditor is a whole lot more light weight and easy to use. You could say that the goal of WYMeditor is making the creation of semantic content easier, especially for people that's not all tech wizzy.
It's true that you with some modification can turn WYMeditor in to a WYSIWYG editor, and some people like that - but it is in no way the goal of this project. Same thing goes the other way around as well, It's possible to use and/or modify most WYSWIG editor in a way similar to WYMeditor as well. Some examples of that include the post editor in WordPress and editors such as widgEditor and WysiHat.
Cheers!