I spent part of the weekend playing the campaign on Hard. I heard that matchmaking was borked, so I just kept to the campaign. It's mixed so far. On one hand, I was not too keen on Blizzard changing the campaign much, but I still had hoped they would increase the difficulty.
-With Culling of Stratholme, I feel Blizzard unintentionally made the mission even easier. Provided you leveled Divine Shield to tier 3 (45 second immunity, 60 second cooldown or 15 seconds without bubble), Arthas can solo-kill Malganis. Since Malganis can barely collect zombies, it makes the mission rather simple. Yet, Blizzard changed it to have Malganis also drop Robes of the Magi, Health Stones, and other +6 attribute items each time you kill him. With these items, Arthas can steamroll the map for the most part. One change I welcome, however, is that once you kill Malganis his forces immediately rush your base, which added some excitement to the mission.
-Fall of Silvermoon was mixed. I enjoyed the inclusion of new elven heroes to fight, but the AI needed to be more aggressive with unit creation and/or the ability to resurrect said heroes. Absent those changes, you can easily just grind them down. Although probably not optimal, I loved just spamming Unholy Frenzy on mass Crypt Fiends and watching everything die.
-The last Undead mission was decent. The final rush was larger than I remember and it really came down to how well I managed the prior waves.
In terms of bugs, the campaign has been smooth (so far...), although some of the animations could use a touch-up. The menu screens are weird though, between weird load times and inconsistencies.