The world of 40k is pretty daunting at first, unless you're lucky enough to go in with a very set idea about who you want to be. For me it was pretty easy; I saw the first plastic boxed set of beaky Space Marines and i was hooked. It was a close thing, because right there next to them was the plastic Space Elves boxed set. How things could have been different...
But like everyone, the universe changes and evolves (devolves if you ask some) and slowly you question which army is for you.
For me it could never be Eldar. The minis are great, but they're so angsty, like every anime teenager all rolled into one civilisation. Orks, too messy and played for laughs mostly. Choas, yes to spikey Marines, but no to all that mutated crap. Imperial Guard, very cool, but the last Codex basically made them cannon fodder with no skills - a terrible depressing backstory that makes it a no from me. Space Marines, I literally have hundreds of them, but more and more they seem like xenophobic dicks. Sure, that element was there in the Deathwatch & Inquisition, but Space Marines always seemed so above that stuff - not any more.
Dark Eldar - big fan of the look and feel, but again, could you ever really side with them? They're slave traders!
And then there were the Tau, the new kids on the block as far as the universe goes. Always liked the look, and the more I read about them, the more they struck accord. To my surprise, it's Tau I've found myself embracing. Tonight I found myself basecoating a drone, just really happy to add to the army - not a chore or another piece to a never ending puzzle, but part of something to identify with and enjoy.
There is some crticism of the Tau out there, called space Communists and so on, but really - in sharp contrast to the bitter autocratic xenophobic Imperium - where would you rather throw your support? And for me, that's it - this may be a fictional universe - but why not find a faction you can be totally happy with? Luckily for Space Marines you can invent a more caring and sharing Chapter, which most people seem to, and bumble along. Im not going to give up on Space Marines because they are very cool, but finding the Tau was a real eye opener and a total surprise.
Do you play a particular faction for a reason other than their look? Let us know...