I think Heavies will be ideal for "escort" mission types, whether that be an individual (play Medic here) or machine-thingy (play Engineer here).
In one of the preview videos (the long interview from the the Utah ski lodge) I thought it was pretty fun when a heavy mowed down 5-6 guys in not much more time than it takes to tell about it. Pretty impressive. Heavies *will* have their moments, but also their frustrations, me-thinks.
I did see that vid, and in that scenario being a Heavy would be fantastic.
However, from what I remember, he enjoyed the benefits of attacking from the flank while the enemy was distracted with reviving their teammates and attacking the maintanence 'bot, and then he held them off at a choke point. Overall, he took about four bullets during the fight, and it was only at the end when he got into close-quarters with an enemy while being exposed to their fire that he finally dropped to half health.
He was, like I postulated, guarding the objective, which is the best thing a Heavy can do, as it is arguably one of the few things they can do extremely well.