That depends. Mount Hyjal and Black Temple were being cleared with 15 to 20 fully Sunwell geared players right before Wrath came out. I did a number of these raids myself. They were more difficult, sure, but when you had 15 or 20 players that had done the fights hundreds of times for a year, well, it's really second nature to many folks.
Sunwell was a different animal due to some of the DPS / Tanking / Healing checks throughout the instance. You'd have been hard pressed to do Brutallus with anything less than 22 people – and even that would be pushing it for most top-of-the-top-end guilds (pre nerf).
If you are going in with people that truly have been there for a year or more, I would expect it could be 10-maned easily. If the players are really top notch, the kind of god like players that you kind of wonder if they're cheating they're so good, you could probably see a 5-man clear MH/BT. For everyone else I'd recommend between 15 and 20 players.
Kara, Gruul, Mag, SSC, and most of TK can probably be 5-maned without too much hassle. I would say however that with Kael'thas you might run into significant problems. And remember too that several fights have legitimately hard mechanics which require extensive study and practice (ie: Kale'thas and Vashj). Even though the bosses might not hit hard, the actual encounters themselves still require coordination.
"While I understand it's called WoW Insider, I'm curious on what everyone at WoW Insider thinks about the upcoming Blizzard releases of StarCraft 2 and Diablo 3? Also, any plans for a StarCraft Insider and Diablo Insider?"
Everyone here is excited about both those titles, and we're gonna be first in line to get them. I personally had the chance to sit down and play them for several hours at BlizzCon thanks to the press room, and I can say that they're truly amazing games.
I don't think there are any plans for specific sites in the future, but I can't really say one way or another. We do already have excellent coverage of StarCraft II and Diablo 3 over at JoyStiq and Big Download. For some more coverage, check out the interview I did with Chris Sigaty (lead producer of StarCraft II) and some gameplay impressions of StarCraft II. Also take a look at Daniel Whitcomb's coverage of the Diablo 3 Class Panel and Diablo 3 Art and Lore Panel.
Have questions about the World of Warcraft? The WoW Insider crew is here with The Queue, our daily Q&A column! Leave your questions in the comments and we'll do our best to answer 'em!