First, I give the Lan gameplay a 9, and I can't imagine how good it will be with future patches. But it's a lot of little things added together that are causing lower reviews.
I didn't need a turorial (although I like them) and figured out the game slowly. My friends do need one (and I mean they really NEED them). And I would just tell them to read the rulebook, but it was pretty darn skimpy too.
Campains/Story line makes a sandbox game into a "world", I played the Starcraft and Warcraft III campains both at least 3 times, and that's what made me excited to try Wow (in fact, there would be no WoW without the campains in Warcraft 1-3).
No game replays. Now I love replays, they are like sports shows to us. A learning tool; Watching top players battling; a hoot to watch after you just played an epic game with friends.
Lack of maps. There are "enough" for us, but, they do seem to be very few, and many people love all sorts of maps.
Problems with online MP; they are working on it. It's sad it happened. Lets hope word of mouth counters the bad reveiws when MP is fixed.
Whether you don't care for whats not included in DG or not, lots of people do like many of the things that where not included, and it just all added up. I'm just lucky I mainly care about Lan, which is fantastic.