I think it is a bit early for a more formal "postmortem" on Cataclysm's first tier of content, but for posterity here is a screenshot of raiding progression as it stood at nearly 4am Tuesday morning, before the numbers could be "sullied" by the 4.2 nerfs.
Since there is no 100% boss, some reverse engineering of WoWProgress's numbers shows that there was a total of 62,405 guilds that killed at least 1 boss this tier. A further breakdown estimate goes something like this:
1/12 - 62,405 - 100%
9/12 - 44,107 - 70.68%
12/12 - 23,122 - 37.05%
13/13 - 812 - 1.3%*
Depending on how many raiders you associate with a raiding guild (15-30), this means roughly between 589,245 to 1,178,490 players who started this tier did not finish it on Normal. WoWProgress pulls its data from NA, EU, TW, and KR servers, which comprise roughly ~6.5 million subscriptions per MMOData. This means that at the upper end (30) the raiding pool this tier is about ~28.8% of all accounts. Or, 71.2% of all subscribers did not raid, and of those who did raid, 62.95% did not kill all 12 normal mode bosses.** In this context, seven bosses in Firelands may almost make sense.
The other thing I want to mention briefly is that I expect Blizzard's Q2 investor call to either look absolutely amazing, or completely terrible depending on timing. As you may or may not have heard, Blizzard sent out emails to existing accounts which essentially contains a free copy of the original WoW game, 30 days of game time included. Secondly, Blizzard is poised to release Cataclysm in China July 12th. Finally, and perhaps more earth-shatteringly from a subscription standpoint, Blizzard increased the Recruit-A-Friend XP bonus from 1-60 to 1-80. If you are an alt person as I am (or was, considering I have a full 10 character slots on Auch), this is about as close as Blizzard seems willing to get to letting you buy a Cataclysm character. Back when RAF originally came out, I had two instances of WoW running and essentially spent $5-10 to get a level 60 rogue, priest, and hunter (with the gifted levels) in about two weeks of leisurely play. And for that month, for all intents and purposes I was two subscriptions to Blizzard. Of course, Blizzard recouped $25 or whatever it was when I decided to transfer the RAF priest to my primary account before shutting the RAF account down.
So, basically, depending on when the Q2 sub numbers are compiled Blizzard will likely be seeing huge growth (due to 4.2 being released, dual-boxing RAF accounts, free copies of games going out, new expansion in China) or further drops depending on when the numbers are locked in for the report.
*Apparently Heroic Ascendant Council is more difficult than Sinestra based on number of guilds having killed it: 812 vs 926 (Sinestra). It might be that people were racing for Sinestra kills before the patch, but it is interesting nonetheless.
**By contrast, only 42.32% of raiders who downed Marrowgar did not also kill the Lich King. It is entirely possible we will see more 12/12 after an equivalent amount of time has passed, of course.