I've loved playing pet classes in any dungeon crawler I've played. Diablo 2, Torchlight, etc.
However, I'm always turned off by the clinically retarded pet AI. Pets getting stuck on any sort of mixed terrain, can't path properly in narrow passages, don't follow you properly when traversing long distances, get lost randomly, etc.. Pet Necro was the most frustrating class I played in Diablo 2, especially when you went to the Arcane Sanctuary with all the narrow passages in Act 2.
Been seeing some playthrues on Youtube, the new pet class "Witchdoctor" or something like that, can summon rather many diffrent creatures. As an example three dogs, frogs who explodes, Firebats who set mobs on fire. And some more, looks awesome, and are working as tanks and DPS.
Bumping for some answers from the people lucky enough to get into the beta. I love playing my summoner necro in D2 but the AI is just so horrible. It isn't just the narrow passages like the maggot lair either, literally everywhere you go unless you have an enigma is just a pain to get them to keep up and do your bidding. So far the Witch Doctor looks like my #1 pick for a character but I will refuse to play it if the minion AI is similar to D2.
They've said that mob AI is vastly improved since D2. They've said that mobs will know to go for the "squishy" one.
I severely doubt that they'd build this massive AI improvement and not dump it into the pets too.
I can't say for sure of course because I'm not in beta, but I'm pretty confident that you won't have to worry.