Basically, in almost any way you can think of, the Eloi and the Morlocks are characterized as opposites. And they're not really unique individuals. If a character is an Eloi, you can bet he is a harmless idiot who's ready to believe that people could fall from the sky (2.5). If a character is a Morlock, you can bet he's only interested in eating – and maybe keeping the machinery of the world running and clothing the Eloi, probably out of "an old habit of service" (7.2).
If a character is a human from the 19th century, all bets are off. The character might be as dumb and lazy as an Eloi (like the Provincial Mayor). Or the character might be a mechanical whiz and meat-eater, like the Morlocks. (Hey, doesn't that sound like a description of the Time Traveller himself?) So, let's just say that 19th century humans come in all shapes and sizes.
The Eloi spend their time making flower garlands and flirting by throwing flowers at each other (5.34). The Morlocks spend their time stealing the Time Machine and hunting the Eloi. So, what does that tell us about these two species? The Eloi are associated with beautiful, delicate objects and things that are considered frivolous. The Morlocks are associated with villainous acts (like stealing) and violent acts (like hunting). So the Eloi are weak and harmless; the Morlocks are underhanded and dangerous.
Also, check out the clothing they wear: the Eloi wear tunics, belts, sandals (which reminds us a bit of Ancient Greece or Rome) while the Morlocks wear nothing (3.13, 5.32). So while the Eloi are associated with a fallen civilization (civilizations that might be associated with decadence) the Morlocks are associated with no civilization at all. The Morlocks look like apes (5.27) or worms (6.1), while the Eloi are "graceful children" (5.33).
What does it tell us about the characters in the novel that the Time Traveller lives in a house in Richmond (a suburb of London at the time), while the Eloi live in communal houses and the Morlocks live underground? This probably could be turned into a whole paper topic, but here are some ideas.
The underground is often associated with unpleasant things, like worms or ghosts, which are two comparisons the Time Traveller makes when talking about the Morlocks (6.1, 5.27). Let's do a little thought experiment: imagine that the Morlocks are still monsters that eat the Eloi, but instead of living underground, they live in the trees. Well, then we might associate them with apes, and we'd want to avoid forests, but they become a little less frightening and supernatural, right?
Whereas the Morlocks' underground lairs make them seem more dangerous and disgusting, the Eloi's dilapidated communal dwellings make them seem more weak and less individualistic. (In fact, both the Eloi and the Morlocks seem to live in communal structures, so it's not surprising that there aren't a lot of unique Eloi and Morlock characters running around.) If the Eloi lived in individual houses – or even individual apartments – then we would say they're like 19th-century people who have a sense of personal space. Instead, they're described as sleeping together "in droves" (5.25). And the fact that their houses are dilapidated emphasizes (again) their laziness.
Finally, what does the Time Traveller's suburban house say about him? He seems to be a bachelor, but he's got enough space for a laboratory and money enough for servants. That tells us that he's upper-class. His life of leisure might associate him with the Eloi – except he's different from them in that he lives alone. He's also different from the other 19th-century humans, for that matter. Whereas the unnamed narrator and the Medical Man meet at a club, there's no evidence that the Time Traveller gets out much. He seems to prefer staying home doing his solitary experiments.
Occupation is one of the few ways the 19th-century humans are differentiated from each other. In fact, it's almost the only way. We don't think we could tell the Medical Man and the Psychologist apart if they didn't have (slightly) different occupations as names. The same goes for the Journalist and the Editor, who think and talk fairly similarly.
By contrast, it's easy to tell the Medical Man from the Journalist. The Medical Man is skeptical yet interested in this whole time-traveling business. When he hears the whole story, he expresses some concern for the Time Traveller's health (12.24). In other words, he seems pretty much like your average doctor – interested in health and able to follow a scientific conversation, but not a theoretical physicist. On the other hand, the Journalist doesn't really care about time travel one way or another; it's just something for him to joke about and possibly use as fodder for an article (2.11-12). When the story is over, he's not thinking about what time travel might mean for humanity or even concerned about the well-being of the Time Traveller. He's only interested in how to get home that night (12.16).
We could group most of these occupations according to their scientific awareness. Some of them know about science (the Medical Man, the Psychologist, the unnamed narrator), some know nothing about science (the Provincial Mayor), some only see science as a way to make jokes or sell papers (Filby, the Editor, the Journalist), some don't really speak much at all (the Very Young Man, the Silent Man).
This is a long way of saying that one of the big distinctions among characters is how they approach the central scientific idea of time travel. None of the guests (except for the unnamed narrator) really believes the Time Traveller, but they all reject the idea of time travel in different ways. Filby is argumentative but doesn't seem to care about the outcome of the argument ("You can show black is white by argument [...] but you will never convince me" (1.33). The Medical Man seems more interested in the truth of the argument. And when the Time Traveller makes his model time machine disappear, the Psychologist offers an explanation. It's hard to imagine Filby doing that.