Recently, I've been looking at new stuff that's been coming out, and/or whether or not I really want to put the time, effort, and money into acquiring the army. As such, I figured I'd share some insights I've had on the process, since we all tend to come to the army decision a little differently.
They Have Cool Fluff
Honestly, the 40k universe (...at least for those of us that are into it, y'know) is a cool place. Granted, I'd not want to live there unless I was working for the Tau, but it's a neat setting. It's old, it's well-developed, and there's lots of room in the fluff for just about anything you might want to do.
Most of the armies have a very characterful background, and sometimes this is what draws a player in. You can have an army full of epic heroes or villains a la the space marines and their chaos step-siblings, all the way to an army of faceless guys just doing the best they can with what they have (IE: IG and Tau). There are monstrous, soulless things that go 'bump' in the night like the Tyranids and Necrons. There are the inscrutable Eldar, the psychotic Eldar, and the fanatical Inquisition troops.
Bottom line? If there's a role you want your army to fill in the grand scheme of the universe, you can probably find one.
Damn, That Unit is Cool!
Sometimes, you thumb through a codex or other info source, and you look at a single unit and think "Man, I'd build an army to field that." For me personally, it's looking at Thunderwolf Cavalry and the Lone Wolves from the new SW codex. Let's face it, some units are simply awesome in terms of theme/looks/what-have-you. Dreadnoughts also do it for me, but it's really a person-to-person thing. Same goes for Grey Knight Termiantors, but I suspect WS5 + S6 power weapons = giddy for most anyone looking to beat someone down in hand-to-hand.
Some codices lend themselves to certain themes. If you want to do an all jump-pack army, for example, you can get away with it in the Blood Angel codex. If you want to do expensive, killy bikers, you can look at the SM, Eldar, or Ork codices.
This also covers the more 'mundane' themes like horde (I don't think you do it with Marines that often) and 'small elite army', or 'mechanized' or 'speedy little boggarts in annoyingly hard to kill transports' (Eldar.). Some of the more 'mundane' ones are also practical concerns; I'd kicked around Guard 'til I realized I'd be painting like 50-100 infantry.
Some codices have cool models attached to them. It's as legitimate a reason as any to decide you want to be able to build/convert a unit, and ties in with the 'cool unit' or 'army themes.' It also has to do with the codex's abilities; I could very well see someone doing something like a horde of HR Giger's xenomorphs with the 'nid codex, or maybe a Gears of War theme with marines, or a Robotech theme with the Tau, for example.
Codex Play Style(s)
Play style goes a bit with theme. Fact is, armies play different. (surprise, I know) I mean, if you really want a bunch of skimmers, you should probably not play the Chaos Marine codex, and if you like vehicles you really won't get a lot of them out of the Chaos Daemons codex. Each codex has a style or styles, and if you aren't into them, you aren't into the army, pure and simple.
On the other hand, the newer codices tend to have lots of viable options. The IG and vanilla SM codices, for example, have a host of options and most of them are pretty viable. (Marines get slops for Vanguard vets, and the IG kind of hurt for decent elite options outside of the Psyker Battle Squad). You can stay with one codex if it's versatile, and build up a host of models that let you swap playstyles from week to week.
Alternatively, you may get into a new codex simply to try a new style and grow yourself as a player. It's why I moved from SM to Eldar, as they are kind of night and day in terms of codex styles.
I expect this one to get the most outcry, but it's as legitimate a reason as any in this hobby. We're in a wargaming hobby; we want our army to be able to kill people and take their land. It's hard to really say which codexes are absolutely powerful, but some codices have more limited effective builds (IE: older ones like Tau, Eldar, Dark Eldar, Daemonhunters and Witch Hunters) and are sometimes simply deficient in some capabilities (Orks, Necrons, and the current Tyranids have issues with killing vehicles, for example).
On the other hand, you have to combine an army's lethality with experience. I mean, I could give a child an AK-47 with an underslug grenade launcher and make them fight a caveman at 20 paces, and while the child's much better armed I think the caveman might still win. You need not only the tools, but the talent. Sometimes this can make crappier armies work against better ones, but then it's the generals in action, and that gets beyond the scope of this article.
At the end of the day...
Most people make their end decision based on multiple factors listed above, and/or some I haven't listed. I'm sure money's always an object, but that's just the game we play. (It's like the joke about playing a well-dressed guy lugging his army in a shoebox: he's playing Grey Knights or Daemonhunters). I'm curious to see what's driven some people to their armies.
On My End...
Space Marines (former)
They're a forgiving army to start with as they're competent all-rounders, but they're also difficult to get right because they're all-rounders. Ultimately, I got sick of painting power armor, as most of their models are guys in power armor with slightly different loadouts.
I may restart them, but again they just had some issues that good players could overcome. I'm a big fan of the theme of aliens (I mean, I saw the movie at like age 6-8, and have a poster of Giger's Alien sitting over my bed...) but they either win big or go home. Plus, I got burnt out painting up all the little ones.
I wanted a challenge to play. I didn't realize I'd be committing to buying a crap-ton of Destroyers and such to even have a chance. The heavy metal range also was a turn-off; it drove the cost up for an army that's in dire need of a new codex. Some folks note that "More People Should PLay So You Can Get a New Codex," but I'd in turn challenge them to drop $200-300 on an army that's unreliable and outdated.
Daemons (Currently inactive)
Modeling theme says it all. They've got issues, but their big factor for me is modeling opportunities and...mostly modelling opportunities. Plus, it helps to be able to laugh off the dice killing 500pts of your army before it arrives due to terrible deep strike luck. (Yeah, 12" into my own guys, followed by 'DEATH' on the mishap for fateweaver. Thanks a lot, dice.)
Modeling, fluff, and power drew me in. They do flow towards a more limited set of builds, most of which are mechanized. On the other hand, I think the 'dex can hold up well, and the Seer Council is a joy to model.
Rail guns, battlesuits. Enough said. They're effective when done right, but their codex needs an update to give them variety in their builds.
Space Wolves (prospective)
I like some of the units, but it'll depend on time and money as to whether or not I do them. It'll also depend on whether or not I can execute a theme I like; I've got the itch for an army that's more fluffy and still somewhat effective. Plus, I want to be able to lavish some attention onto the models.
If the new codex is decent and the new models are cool, I'd be willing to try to pace myself and do the bugs again.