For example, quite a few people have already been complaining about joining campaigns where the rest of the party was several levels higher than they were, without realizing it. Others have noted that if you join a group already in progress, your journal isn't automatically updated to reflect theirs. And reports are already showing up of PKers hanging around entry zones and killing any newbies who show up.
As someone else pointed out, to really make this game shine will require more than simply DMs: it will require extremely good mod creators who can plot originally, write effectively, design intriguing battles, and present the whole package with an eye to balance. This would seem to be tailor-made for someone like the folks at Team-BG, but it remains to be seen whether any bright talents will appear in the near future. I strongly hope so.
Have you had any experience with multiplayer, yet?