Old Shas gave me strong Grey/Sectoid vibes, which I liked. Plus the meme camo shell game playstyle was pretty unique outside Ariadna.
I love the look of the MAF. They look vicious and hard as nails. Aesthetically they drew me in. I've always been more of a painter/modeller than a gamer and Infinity is pretty crunchy and intimidating so I found the very direct approach of the MAF better suited to my skill deficit. I was totally sold once I read the army lore. I'm a Star Trek fan as well, enjoy the Klingons and saw parallels. No angry fan boy alarms here. There are few original ideas in sci fi and seeing someone evolve an idea is awesome. The Morats strike me as part Klingon while their relationship with the EI is a bit Jem Hadar/Dominion. While the elements are there CB took the ideas, made them their own and seamlessly integrated them into their own universe. Awesome stuff.
For me, it's always aesthetics first. The Anathematic and all of the badass looking TAGs, the Umbra, Sygmaa and the Oznats really drew me in. On the table i love having access to all the toys along with our unique ones, like my beloved sepsitor. I really enjoy their fluff and the enlightened genocide they exercise. PanO was my go-to faction before, but i've converted heart and soul to the Combined Army...even though i'm taking an NA2 break with Druze...
I don't actually play the faction (or any faction), but I like the CA because of the Morat. Big angry space oni that basically follow the strongest. It's funny to think that if humanity gives them a big enough beating, they'd switch side against the rest of the CA!
At first I choose TOHAA, but my friend was first and he ordered his first minis. I did not wanted to play mirror matches, and in my area, there was no "The Bad Guys" player as far as I knew. So I joined dark force :D