it'll depend on the healer and the tank ofc, I do actually remember the heroic mobs hitting hard, for example, the packs of mobs in slave pens that patrol around at the start, any one remember those collectively gibbing tanks? i do. underbog heroic, the twin giants, kinda remember that being untankable at the start, one of them had to be kited. and so on and so forth there are various wipey pulls. but the tuning of the heroics at the start was brutal compared to the gear most ppl had, t3 might make it a bit less one tappy, but I still think there are going to be packs of mobs that will simply one shot the tank if you try to tank them all. at least until you have t5 and the armor value to mitigate a bigger % of the physical damage.
mobs in steam vaults for example, I remember some of those patrols being OP, the ethereals in mana tombs hit exceptionally hard for some reason, low geared tanks could die easily to these mobs without CC you get pummelled.
compared to classic, you don't get one shotted in dungeons, you would have to ass pull a whole pack of mobs, if you simply aggro a single mob, or several non-elites you can survive easily enough no matter what class you are. but in heroics, if you pull aggro you'll probably die. thats the difference. if you aren't the tank you'll get one shotted by most of the mobs in heroics. you might survive a swing or two but if they crit you're dead.