I just started playing 40k a couple months ago and I have 1 complete army with 2 others in progress.
My CSM World Eaters are my complete army. I picked them because I think their paint scheme looks cool and I liked the idea of a bunch of crazy dudes running around growling and snarling and killing stuff (Oh berzerkers how I love you...). The squad of oblits and a couple defilers just add to the killyness.
My CSM Thousand Sons Army is almost complete. Just placed my order for Ahriman and another squad of termys then its done (Except for paint. Haven't even started that yet). After I read these guys' fluff I went out and bought 2 boxes of them right then and there because I thought they were so cool.
Last army is Space Wolves. Just started buying models for these guys. I've always been way into Norse Mythology and they're the next best thing to actual vikings in 40k.
Also thinking about starting an Eldar army so I can play something other than marines if I want...