Last night I ran Grim Batol heroic. It was with my guild so it was a blast. What made it so? For three of us it was our first time. We cleared it in 45mins. On two bosses we were three-manning it. (I was one of the dead one of those times!) Why was it fun? We didn't know the fights but figured them out on the fly.
What else has been fun in heroics? It's usually the unexpected.
I had to train-trap a big ugly heroic mob for four minutes while a pug tank fixed his router.
With only me and a healer up (who tried to dps), I kited a different big ugly heroic mob around while widdling its 1M health down.
I got called into heroic Deadmines while doing my TolB'd dailies. I unfortunately still had my gorilla. After the tank went down to the whirling blades of the robot in the forge the healer somehow thought to heal the pet. I threw on growl and MD and the damn pet took hardly any damage.
Being SV, my only boss interrupt for the first boss in heroic Shadowfang Keep is to use my Gorilla and pummel up to every 30sec. I was astonished that I was able to do it. Pummel works a lot better than it once did.
In general, the heroics feel like five man raids. Like raids, pug raids can really really suck. This is magnified by pug players that you will never ever encounter again from other servers. But if/when you have a good group the runs challenge your abilities and certainly keep you awake.