It's hard to put my finger on what I didn't like about DL magic...it's as much tied up with the characters of the various books as it is with the mechanics.
I didn't like everyone wearing a robe that announced what their allignment was, nor the fact that there was a single governing body of mages. I didn't mind the phase of the moon stuff, in fact I thought that was rather ingenious. Even more than the magic system, is my dislike of Kenders.
Well, not Kenders per se, but the shallow way in which they are portrayed. I felt like they took one character and said all Kenders are exactly like this, except some are female.
I don't really dislike the DL setting, it's just that given the option, I would choose FR over it. I guess that's the same thing with Greyhawk as well. But with GH, I just don't know anything about it (though Manikus the Ranger was born on Oerth).
Upon further reflection I think some of my issues with DL are related to the published art, some of which I don't like (but there is plenty of it that I do like).