I almost see less of a difference in most sci-fi between species than races in D&D. Then if they happen to have a culture different than ours, there's something "wrong" that needs to be fixed. Well, that and they all have a single mono-culture or at most a single rebellious faction. The rebellious faction are usually the good guys because they're more like us.
Yeah I think that applies both ways also. Depends on the nature of the specific fiction, I suppose. Star Wars, for instance, most of the different races don’t seem to offer much, except a strange language (which everyone can understand anyway) and little else. Chewbacca is the notable exception. There are a few others.
Tolkien, because of the depth of the work, did a good job of making the elves seem different from men. Not entirely alien, but certainly different. Less so with the dwarves, in my opinion.
Most D&D games, when it comes to race, it seems more to me about having a few potential hooks into the game world than about playing something other than human. Like if you play an elf, it kind of dictates where you’re from....usually there are a couple of elven lands and the same for dwarves. Humans are more ubiquitous. So it matters for background and how the PC connects to some elements of the game world.
But when it comes to roleplay, I don’t tend to see people play elves all that differently than humans. They still have the same emotions and behave in very understandable ways. Same with dwarves. Each may lend themselves to certain traits....elves may be aloof or act superior, dwarves may be gruff...but these are still fundamentally traits which we can find among humans.
I imagine this is likely very true of non-human characters in sci-fi games, too.