I dunno if anyone's ever pointed this out. I'm not sure it entirely "clicked" with me until looking at the thread about the Clone Wars TRU figure (Kul Theska?) and thinking, "Man, those aliens are all over the place in the prequel era. Wonder why we never saw them in the original trilogy?"
Thinking about it now, the lack of continuity of established aliens over flashy new designs is a problem (albeit a minor one) with the prequels. Sure, it makes sense that there aren't the same species all over the place, and we see different (and, in some cases, more urbane) worlds in the prequel trilogy. But shouldn't we be seeing more of the established species?
Sure, we have Rodians. Rodians all over the place. And there's a delegation of Ithorians in the Senate, and EU Ithorian, Mon Calamari, and Talz Jedi. And the new guys from the Special Edition Cantina scene (I forget their species). And more bloody Twi'leks than you can shake a lekku at, as well as Jawas and Tuskens on Tatooine because, really, how else would we know it was Tatooine and not some other "farthest from the bright center of the galaxy" desert farming world?
But... Snivvians are one of the only species (I'm actually thinking the only species) seen in all three original films. By that logic, they should be really common. Yet there's not a single one in the prequels. Same goes for the Saurin, Ugnaughts, and any other alien species seen in at least two OT movies. (We're seeing the outskirts of civilization; shouldn't alien species that have colonized boondock planets be seen in the center of galactic culture?)
On the other hand, there are a boatload of new species who are so common in the prequels, their absence from the original trilogy makes no sense. Dugs, Togruta, Zabraks, whatever the hell Ratts Tyrrell is. Those aliens seem to be so common, you can't turn around without bumping into one. Why are they totally absent from the original films?
I dunno.... Like I said, I know it's a minor point, but it seems like we should've seen more classic aliens, along with a lot of new ones who weren't seen in such numbers that they seem like some of the most common in the universe.
(And the Aqualish don't count. Those things in the prequels are the same species as Ponda Baba. I can buy the whole "two species, one with flippers, one with paws" thing, though it's pushing it. I can't swallow a subspecies with six eyes and a totally different facial structure that exists only because some costume designer was showing off.)