We've heard the Night Elves talking about Elune since Warcraft III, but she has always been a kind of odd figure in Warcraft lore. Generally speaking, the good gods or god-like beings are the Titans - but the Titans aren't really directly worshipped on Azeroth. We found out the main reason for this in Chronicle and Legion - the Titan Pantheon has been dead for ages. The Keepers - beings created by the Titans - have also had godlike status for some, though again, the machinations of Loken led to many of the mortal races (who had been transformed by the Curse of Flesh thanks again to Loken) sort of forgetting these figures as well.
Humans do retain the legend of Tyr - a legend that actually dated back to before they were humans, when instead they were Vrykul. The Curse of Flesh had affected them (most Vrykul we see are fleshy) but they had not fully shrunk in size to become humans. There are actually two overlapping narratives of why humans wound up in the part of Old Kalimdor that would become Lordaeron - one in which vrykul whose pygmy children were threatened with infanticide by King Ymiron fled south to save their offspring, and one in which some Vrykul (as well as Earthen and Mechagnomes - the EK members of the Alliance are old, old allies) followed the Keeper Tyr after Loken seized control of Lordaeron.
Tyr is remembered - vaguely - as the exemplar of honor and courage, and his silver hand became the sigil of the first human (and dwarf) order of Paladins. But he's not really worshipped like a god. The Church of the Holy Light has a lot of the trappings of real-world western religion, but the focus is almost entirely on philosophy rather than theology. The Light in this sense bears a much stronger resemblance to the Light Side of the Force from Star Wars than an Abrahamic deity.
What's also interesting is that in Chronicle, there is a passage that say human spiritual leaders started having visions of thrumming, geometric embodiments of the Light, and built the Church around these figures. While not explicit, it seems pretty obvious that they were visited by the Naaru - meaning that there were Naaru on Azeroth, or at least communicating with those on Azeroth, long before the Draenei showed up.
In a sense, Draenei have the most straightforward and practical religious beliefs. They see the Naaru as saviors because the Naaru literally saved them from Argus. And it's not like the Naaru just mysteriously vanished into the aether after this miracle - the Naaru stuck around, and have been hanging out with the Draenei for thousands of years. Even if they had left, we're not talking about generations going by with only legends passed on. Many if not most Draenei alive today were born on Argus and have absolutely no reason to doubt the power of the light or the benevolence of the Naaru.
The Tauren only recently developed a form of Light worship, and this is bound up in a reverence for the sun, which they call An'she. Tauren belief is largely based around the monotheistic worship of The Earth Mother. Now that we know that Azeroth is itself a nascent Titan, I have to imagine a lot of Tauren are feeling that their beliefs have been fully vindicated. Azeroth, the Titan, is the Earth Mother. Granted, for a Titan she's basically still an embryo or a fetus, but she's been around long enough (and is apparently already intelligent and able to communicate, if we think Magni's story is true) that she can still claim a sort of Gaia-like mother-ness to the living creatures of Azeroth.
What's curious is that the Tauren consider the moon and the sun to be the "Eyes of the Earth Mother." They refer to Azeroth's big, white moon as Mu'sha, while they call the sun An'she. Oddly, there's no mention of Azeroth's second, smaller and bluer moon, but then, there's actually not a lot of stuff that refers to that anyway.
Mu'sha is basically just the Tauren name for what the Night Elves call Elune.
Clearly An'she grants them power - he has allowed them to have Paladins and Priests. One could argue that their belief in An'she as a godlike entity doesn't necessarily need to be true if it gives them the faith to call on the Holy Light, but I do think it's something worth considering.
But then we have to come to Elune. Elune does not obviously fit into easy categories, and the more we learn about her, the more confusing it gets.
First off: she's not exactly hidden. Elune is literally Azeroth's bigger moon. Maybe. It's possible she is an entity that inhabits this moon, or even simply uses the moon as a kind of avatar or a way to manifest her power.
There are hints that she might be a Titan, but also a lot of evidence that suggests she isn't.
On the pro side: One of the five Pillars of Creation is called the Tears of Elune. The other four are all named after known Titans, and so if she's not a Titan, this Pillar would definitely be the odd one out.
She is also associated with a celestial body. A moon is a lot smaller than a planet, and also, there's a question to ask about whether Elune should be old enough to no longer be kept within a body like that. But still, given that we now know that Titans are basically living planets, or at least beings that emerged from planets, having Elune reside within the moon is more or less consistent with this idea about Titans.
She is clearly incredibly powerful.
And here's a subtle one: Titans are associated with the Arcane. While Malfurion and Tyrande's Night Elf society was one that, until Cataclysm, rejected Arcane magic, one should consider that a Druid's lunar-themed spells are Arcane, and that the very water that transformed them from Dark Trolls into Night Elves was the blood of a Titan. They might not like the recklessness of Mages, but the Arcane is woven deeply into their culture, and a lot of it comes from Elune.
But on the other hand: We know about the Titan Pantheon, and she wasn't in it. And it sure doesn't seem like Elune suffered from Sargeras' massacre of said Pantheon.
There's also the fact that she almost seems to pre-date the Titans. Aman'thul was the first to emerge from his planet, and he went around cultivating other Titans and safeguarding their worlds. But Elune seems to have been hard at work really at the moment the cosmos began to take form. The huge revelation we got in Legion is that Elune is the one who created the Naaru.
Now, that's also pretty interesting. The Naaru are embodiments of pure light, and while the Titans are generally considered good guys, they were more associated with the Arcane than the Light. There were some examples - Tyr seems to have used the Light to fight against Loken and Yogg-Saron, and we also see Norushen (a Watcher - kind of one rung down from the Keepers, also found in the Halls of Origination) use the Light to purges us of Sha corruption in the Siege of Orgrimmar raid. So this doesn't really rule it out, but it does make you wonder.
And here's maybe the weirdest question about Elune:
If she's so important, why is it that only the Night Elves seem to worship her?
You could argue that the Tauren have some beliefs associated with her, but why is it that the descendants of the Titanforged races don't?
Elune could be a Titan, but if she is, she's a very unusual one.
But perhaps it's more likely she's something else entirely. She created the Naaru, but the only culture that we know knew about the Naaru before very recently are the Draenei, who were rescued presumably light years upon light years away (we don't know how far all of this extends into the Great Dark Beyond, which is Warcraft's name for just normal Outer Space, so for all we know Argus, Draenor, and Azeroth could be in different galaxies.)
We know that the Void has beings known as Void Lords who are even more powerful than the Old Gods (they're the creators of the Old Gods,) and so it seems only fitting that there could be beings that are a Light-based counterpart. Perhaps, in fact, the Naaru serve a similar purpose to the Old Gods, only to save rather than corrupt (though their light-dark cycle demands its own whole post.) Maybe Elune is actually a "Light Lord" ("Light Lady?")
But then, if Elune is so cosmically powerful, why is she associated only with Azeroth's moon? Yes, Azeroth is special, thanks to the extra-powerful World Soul within it, but if Elune is literally the substance, not just the embodiment of the Holy Light (or at least one piece of it,) how could she be contained by something as small as Azeroth's moon?