MC
I didn't solo, but about 10 of my guildies got together to get some of the old world raid achievements and to get a little gear for a couple folks who were leveling alts. I brought mylvl 80 Prot Pally/Holy Priest/Arcane Mage team to MC. I set up as a sort of "off group" handling my own packs of ads while we were clearing trash, and then just contributing DPS / heals when we were on the bosses. That one boss that "set us up the bomb" was a bit of a pain - I just parked my pally and mage out of range and solo-healed rather than dealing with the possibility that I'd "train the raid".

The final boss (Ragnorok or something like that) did that horrible pushback that split my team so far apart that I ended up forgetting about the pally... just concentrated on healing the others and had my mage spam arcane misiles.

I'd say that I could ~probably~ have solo triple-box cleared it, but it would have taken a while.

Maybe I'm just doing something wrong, but I find that mechanics that cause my toons to get out of range of each other really throw me off.

AQ 20
I four-manned (my triple box team and one guildie friend) AQ 20 once. That one's not too bad except for those darn sand mound thingies that would pop up and silence my priest/mage. Once I realized what was going on, I just let my guild buddy tank in that area and I kept my team light on their feet. On, and those sludge/slime things that "eat" your mele? not too bad once you know not to get close to em

BWL
Brought my triple-box team with about 10 guildies (mostly naxx-geared 80s) to BWL, and we had a TOUGH time with that first boss with the eggs and the mind control pedistal... cleared that stuff okay, but things got dicey cuz we blew all our mana trying to stop the infinite adds that spawn after the fight ends. After that, things went fairly smoothly.

AQ 40
Went with a few guildies... could NOT get past the twins... that knockback thing totally messed up my following and whatnot. The twins probably require way too much coordination and separate action (due to the heall/buff-if-theyre-too-close-together mechanic) to be truly multiboxable, but I suppose there might be a workable strategy out there somewhere.

Onyxia
Soloable by most 80s. Pew Pew ... dead.

ZG
Went with several guildies (5-ish geared 80s plus my triple team, and a couple 65-ish). The place was pretty much a ROFLstomp. I'm pretty sure it's boxable, though you have to watch out on taht one boss that separates your toons over to the side.


Burning crusade
Several of my guildies got together for "bc raid nights" and roflstomped Hyjal, BT, TK, and SSC with about 10 geared 80s. I didn't triple box those because of some of the mechanics involving movemet/positioning. We had more trouble on some of the old 60 old world stuff just because most of us were already very experienced in the BC content, having done much of it in the pre-nerf 70 days. We also did Grul and Mag and I DID bring my triple box team to those. The only difficulty there was the shatter and ceiling collapse in Grul (cept he didn't live long enough for it to be an issue), and that annoying pushback in Mag.

Final thoughts
I'd say that the places where I've encountered trouble (even with multiple geared 80s single boxing) is when folks get the idea that since they're 80s, that old content doesn't need to be given any respect. There are still fights and mechanics that (in my opinion) brute force alone will NOT get the job done. My advice to anyone is to at least read up a bit on the boss strategies / abilities so that you're not just "bull in a china shop"-ing your way around and you'll have a much easier time.