I have to say, I really did enjoy the campaign. It had an engaging story, and kind of a Mass Effect feel to it as you go through all the flavored content and extra things that the game has to offer in Single Player.
However, many people would've loved to have a game that costs half as much and only go Multiplayer. Computer opponents can only be so much fun to play against; people are (usually) smarter and more devious in ways that an AI could never be, and the social interaction of playing online with real people has an appeal. Multiplayer is where the problem lies with this game; despite a 14 year beta test of Battle.net 1.0, Activision-Blizzard (Activision has most of the blame, though) managed to screw everything up that made the first one successful.
As someone who liked to make and help others with maps on Warcraft 3 and even Starcraft, the current system in place seems geared toward (excuse the harsh comparisons) internet stalking, due to forced RealID usage in lieu of a 'friends list,' (effectively, if you want to "friend" someone that you likely met on the internet, you must know their real name; Facebook stalkers will be happy for this) casual players who want to click a button and join a game, and Blizzard being able to take over any custom content that becomes popular. Yes, they can do that, in addition to providing you only 25 Megabytes across a total of 5 maps, with a maximum limit of 10 MB for a single map. Warcraft 3 maps that use code almost exclusively (DotA is a hugely popular one, for example, with only one custom model) have already reached and even surpassed these limits, in a day and age of Google and Yahoo offering "infinite" storage for free email accounts. Yet, we pay $60 for 25MB. Region locking is also a big deal; I have friends that I played Warcraft 3 with for years who lived in Australia, and now they won't even have a chance due to not having a North American version of the game. And playing outside of your region is illegal/bannable, but technically Blizzard could ban your account for whatever reason they wanted; they already have your money.
Removal of other essential features, like being able to host your own maps, LAN connection ability, chat "channels," basic friend's list implementation, '/' commands, decent support for a custom map making community, and just being dickish in general after the success of World of Warcraft all account for huge flaws in the the game, and I don't expect it to get any better; if people continue buying it, they'll think they're doing something right, and will continue to do it.
At least single player is nice. And Warcraft 3/Starcraft B.net is still up.
Some further reading, both of which are well written, and nothing has changed in the time that either has been done:
http://www.the-ghetto.org/content/battle-net-2-0-the-antithesis-of-consumer-confidencehttp://www.teamliquid.net/forum/viewmessage.php?topic_id=127066