So just to get this straight, in other MMOs, if the dps is attacking the same target as the tank, instead of competing for aggro, any threat the dps makes gets added to the tank's threat?
No it's not added. It just means they aren't attacking something the tank isn't actively tanking, so are pulling random things off making it harder on the tank.
The only other MMO I played was WoW many years ago, and from what I remember its trash packs didn't usually come with so many normal and strong adds, it was mainly multiple elites (also the packs weren't so spread out). The fights were more about CC and tanks controlling everything, where as in SWTOR it seems that control of the weaker mobs is being expected of DPS whilst the tanks take the elites damage until the rest are mopped up.
As pointed out though, that is in general. There are quite a few exceptions (Mercenaries in Kaon come to mind, which also made me think about how crap people are at interrupting), which people will also expect DPS to know about, unless the DPS say they haven't been there before or are unsure of what the mobs do.