Let's talk some 40k. Today's theme is: themes. I'll draw on my modest hobby experience here to provide some insight in theming your army.
So what is a theme? It's a pretty broad term, and it's applied pretty broadly too. I want to discuss armies themed around a real world or fictional reference. For instance my army is themed around dinosaurs, but other themes could be the Middle Ages, the pre-Heresy period or Christmas. This excludes armies themed around an actual model or unit type (Vulkan, Bikes etc.), as it's pretty self-explanatory how to do such a theme (I prefer to call it a 'build').
Egyptian themed Chaos Space Marines (Image taken from Bolter and Chainsword's Imhotep)
Deciding on a theme
Picking a theme is usually pretty easy. Some people may even have a theme in mind before deciding on their army! However, ensuring you're making the right choice is important. You don't want to end up with half of a fully converted and painted army only to discover it's not what you wanted after all.
So, what theme to choose? My personal prerequisite is that the theme should be something you're really into; I've been a fan of dinosaurs since age four and have been ever since. Not only will the knowledge you already have on the subject help you with conversion and painting ideas, the passion you have for the theme will be a continuous source of motivation.
To the point where you may find yourself unable to start a new army...
Themes to be wary of
While it's hard to say there are 'good themes' and 'wrong themes', there are themes that should be considered extra carefully before deciding to build your army. First of all the joke themes. Everyone has probably seen the infamous Hello Kitty armies out there, and while they can be great fun to make and play, you really should be sure it's what you want before making one yourself. Games Workshop miniatures are expensive and building an entire army as a practical joke is not something you want to regret later.
Also, keep in mind some people don't like playing against joke armies. While personally I would not object to a game, I can understand the objection to facing pink Necrons every other week.
Secondly, controversial themes. Now I'll be the first to admit the Third Reich and racism are part of our history and can therefore be used as a theme for your army. However, if you're considering this you should ask yourself: do I want to use this as a theme so much, I'm prepared to risk offending other people with it?
Of course, games like Flames of War or Weird World War II feature Nazi miniatures, and there doesn't seem to be much fuss about that. But I think the difference is comparable to dressing up as a Nazi during a WWII reenactement, and dressing up as a Nazi in general. So think carefully.
So with that heavyish ending, I'll quickly follow up this post with examples of how I themed my models, starting with infantry