I dunno if it has fallen, but Blizzard has definitely started to have worse quality. The last game the realeased before SC2 was WoW back in '04. That means in 6 years they only managed to release SC2. What do they have to show for it?
Split campaigns, there is NO reason why the Specter and the Colonist arcs, as well as the Protoss, couldn't have been scrapped and in their place they could have put int a valid Protoss and Zerg Campaigns, none other than wanting to milk you for the money.
Battle.net 2.0, the biggest abomination of online gaming in a LONG LONG time. They removed so much that it's kind of sad, on the up side though, they added facebook integration! As it stands, Gamespy back from 2000 is better than this shit. Let that statement sink in.
Multiplayer is a similar, yet dumbed down, versoin of BW. Cut it whichever way you want, the multiplayer is still the very much like BW's, and the fact that I can actually get to the 2nd highest league only means it had to be dumbed down to hell (I certainly didn't get better at this) for this to happen. Then again, if you cater to the chobs and noobs they will praise, so that's why SC2 has been getting so much praise.
Also SC2 cost$60, and Cataclysm is $40. For $40 for an expansion pack I would expect it to come with a model and have her give me a blowjob, because I cannot imagine any expansion packs worth $40.
Diablo 3 is looking pretty fucking good though, they havent made an announcement that will cripple the game, yet.
As for the "your tastes are chaning" argument, go play WC3 or Diablo 2 and tell me if you enjoy them. If you do then your tastes haven't changed and Blizzard has changed. As it stands, as a whole package, SC2 has been their worst game so far, and I enjoy playing any of their previous games more than what they released.