this is a very interesting topic to me. during the time when starcraft 2 was released I really tried to get myself into the new editor with all the cool functionality and features. however, in the first time it was just frustrating to work with it. nearly all "cool" features such as banks, release system, map marketplace etc. weren't working at all. the data editor was just not what I as a experienced warcraft III actually wanted - I wanted to code things and I really didn't want to sticks objects together just like in the more or less hated object editor of warcraft III. I just wanted a cool language with a nice syntax which was powerful enough to handle everything I could imagine to create games. however, this was also pure frustration - as in the warcraft III editor the essential scriting language "galaxy" was/is only poorly supported - blizzard just made a very basic syntax highlighting and that's it. they suggested to use GUI which was better than the wc3 GUI but not the thing I wanted - in the scripting language itself many "features" were still missing when I tried to create maps such as nice dialog functionality and stable timer functionalities. the memory cap was basically a bad joke - even in wc3 I was able to keep more memory around for the gameplay purposes ...
Another really bad thing is the dependency of battle.net as modder for starcraft 2 maps. in warcraft III you were much more independent and could easily publish your maps in several different plattforms which is one reason why warcraft III is still alive btw ... (garena, eurobattle.net etc.)
Besides that most people prefer to create maps and stories for medieval themes just like warcraft and not for galactical interventions such as starcraft 2. The aspect of creating models before one could actually start a cool medival map is just horrible ...
The model section is also not as great as it was in warcraft III because models now take much longer to develop and it isn't that easy anymore to pump out some proper models for games.
All in all I can just say that the first time (first few months or perhaps even longer!) was too frustation to actually continue working with the editor for starcraft 2. I by myself looked for alternatives and found the unity project which was much cooler in my opinion.