Resistance.Halo.Gears.Half-Life. Which is the better universe?

I'll admit to an ignorance on the subject, as I've only played through the story mode of Halo 1(and MP matches of 2 and 3), but how is the nature of the Covenant explained? It makes no sense to me how a bunch of alien species join together under a common religion and base a society around it, why exactly would a bunch of independently evolved sentient races share a common enough psychology for such an arrangement to work? I don't have any problems with the idea of a coalition or society of multiple species, but having that common thread be something of a psychological nature just takes me out of the story(I admit it, I nitpick)

Another thing that seems far too common among these types of games, that I would love to see changed at some point in game storytelling, is the all too obvious human basis for aliens and their technology. How many times has the bipedal alien race arriving in warships that are basically modified modern military designs, with the intent to set up some sort of empire based around the kinds of interests that humans create empires to pursue, been done now? I recognize that having a great story is often not a primary goal for game design, but I would kill for one of these alien invasion games to produce a true feeling of alienness, not just a "humans with a slightly different set of features invading" feel. It's not out of the question, even if they have to rip some ideas off, borrow from David Brin, or Dan Simmons, or Mike Resnick(great storyteller there), or one of the many other writers who manages to set up a backdrop that isn't just Homo sapiens sapiens vs purple Homo sapiens sapiens, someone who will provide a fresh look.

ANeuralPathway

Also, the aliens didn't come to earth first, they encountered each other somewhere else in space. Because the covenant religion is probably based on forerunner lore, they identified the humans as having to be destroyed (I still don't know why). Probably because what was inside earth. In the halo 2 story you really start to see the internal struggles in the covenant. First you go after covenant heretics in a mission, later in the game some sort of civil war starts, and it's brutes vs. elites with pretty much all races choosing a side.