well from what i can say, the heroics are the challenge.
yesterday i did the quest chain and unfortunately was forced to stop at the end of pit of saron due to server crashes.
from what i can tell since i was doing regulars to test out bosses first. is that it was mainly tank and spank by a bunch of 2k dps... me included, my spec was horrible. today i ended in FoS? (first wing) and it was heroic. what struck me is that unlike the other dungeons none of the mobs had a higher level, they all stayed the same. but their abilities were quite boosted though.
first boss actually healed thru non-stop until the gorup figured out how to kill the sphere while th etank was moving around for boss not to catch it up. that fight actually lasted 7.5 minutes and i actually was able to pull two army during the fight. so what i got from it is that heroics are like 5 to 10 times stronger then the regular who could by the way be tank and spanked all the way.
heres my advice, dont do the reg trying to know what the bosses are all about, when you'll end up in heroics, it'll screw you over !