Hey WillieW
I just got done springing "nine tenths" on my party last week. What do we need to do to get you to write more modules????
To your question.
I found as a GM the attacks of opportunity and the segmented combat round to be somewhat tricky.
I also found creating monsters to be more difficult. Old nebulous abilities need to be more concrete for use in the 3e combat round. I used to say things like, "They can fight in ranks". Now I have to figure out reach weapons and facing issues.
I've found deciding on appropriate magic items much more difficult as well. This is mainly due to players ability to create their own items. I'm still struggling to find the right campaign balance between handing them stuff and letting them make it themselves.
I also found the difference in XP progression between 2e and 3e interesting. I was used to lingering at the levels between 5th and 9th. Now, for the same adventures, I would shoot through those levels in 10 to 15 sessions instead of the 25 to 30 I was used to.
As a player I found the narrowness of the 3e skills and spell selection hard to deal with. I was used to the end of 2e in which mages had four 200 page books to select spells from, and priests had lots of flexibility to select their spheres. Characters had lots of skills and you knew them all at max level. Now most priests look almost exactly alike and so do most mages. The skills system is nice, but in practice most players seem to be ending up with less skills then they did in 2e which makes spell casters seem narrower then they were at the end of 2e.