First things first, this article is not intended to suggest that internet 'cookie cutter' lists are wrong, or that anyone that asks for and takes advice from a forum is also wrong. What I am hoping to do however, is to provide an alternative viewpoint as to how you can go about theming army lists without coming up with the default responses.
I'll take a few examples, hopefully also illustrating how and when some of the lesser known units can be incorporated from my own experience. I currently play marines and dark eldar, so examples are currently limited to those armies.
Accepted internet wisdom would tell you that some units/choices aren't worth considering. I say that everything has its place if you want to use it.
Example 1 - Dark Eldar Mandrakes.
Accepted Internet Wisdom - Hopeless, unless you pair them with a Haemonculus to give them the ability to use a shooting attack, which negates their effective use as Infiltrators.
I've used them only once so far, in a game against the Tau, when they got me two victory points early on - First Blood and Slay the Warlord. My opponent was, perhaps quite rightly, more concerned about the raider-born close combat troops that were headed his way and moved up to get within weapons range. my turn arrived and my Mandrakes pounced, wiping out both the commander and his two bodyguards. Of course in the next turn they faced the full wrath of everything the Tau could throw at them and were wiped out, but for 160 points, I'd taken out the enemy HQ and netted myself two VP's that killing the Mandrakes couldn't reverse.
Example 2 - The Chapter Master.
Accepted Internet Wisdom - Only worth taking to get access to honour guard marines.
My marine armies always make use of the Chapter Master. He's 25 points more than the captain for the same stat line, with one glaring exception - the orbital bombardment. The problem with this is that it always scatters the full 2D6", and means the Chapter Master can't move in the movement phase before using it. So it limits your warlord's movement, and only guarantees a hit one time in three. This is where Matt Ward has been quite clever though to my mind, as if you assume it's going to hit once in three, consider its value as 75 points. to get a guaranteed large blast template at S10 AP1. Not much survives that, and it certainly makes back the points spent on it over a period of games. It's actually best used to affect your opponent's deployment, making them spread units out to avoid being hit by the scatter. As noted earlier, it does limit your chapter master's movement, but if you use it in the first turn, that's not so crucial.
So how about theming a list then to get something unique?
Don't take anything you don't like the look of. Not the most original of comments I know, you'll find it in collecting advice everywhere, but if you don't like the models, you won't want to paint them, and playing with painted figures is just better. My dark eldar are a classic example of this, as they contain no coven units at all. Internet wisdom would suggest as a minimum I should be taking haemonculi, but I don't like the models, so they're out. The result is an army that looks very coherent on the table top, with raider mounted infantry and supporting vehicles. It's also an army that fights in a very similar way, with no distractions from using models that don't move at the same rate and get left behind.
Find a way of playing you're happy with,and pick your army to match. There's no point in taking an all bike mounted marine force if you like to have lots of static firepower or outnumber your opponent. Most codices are written with options to tailor your army - the aforementioned dark eldar can go one of three ways using the various options in the codex (coven, warrior or wych cult). Marines and necrons are also perfectly capable of tailoring their forces to a theme.
Write a background story to explain why you're collecting what you're collecting. Perhaps you're an Ork warlord making a mark in a war zone following the death of the former warboss and gradually gathering boyz to his banner, either starting with a few elite troops or possibly just some intimidated simple boyz units with few special weapons etc. (if you've never read any of the tale of four gamers articles, I strongly recommend looking them up and giving it a go). Alternatively, play an escalation campaign, start with 500 points and gradually build your army into 750, 1000, 1250 and 1500 points. These methods build your force slowly and let you identify what areas you want to strengthen, theming your army naturally into a force that works for you.
So there you have it, three rules to follow that will help you collect a very personal force. I'll leave you with a quick résumé of the armies I've currently got and am working on, and the themes behind them.
Theme: light infantry.
Minimal vehicles, largely consisting of bike support, troops units don't usually take transports, with only a minimal heavy support section consisting of elite insertion terminators in a land raider.
Cobra Guard (marines).
Theme: heavy mechanised.
Troops units transported in vehicles, supported by heavy weapon units (scout snipers, devastators) and heavy infantry (tactical terminators, thunder fire cannon etc)
Bloodstone Knights (blood angels)
Theme: in your face close assault &support
Lots of combat units, assault marines supported by death company, sanguinary guard, vanguard veterans and assault terminators. Heavy support comes from fast moving weapons platforms (landspeeders, fast vindicators etc)
Theme: mobility of firepower.
Almost all infantry units in fast skimmer transports, nothing slow moving at all. No coven units thus accentuating the glass hammer approach. Massed firepower supported by some assault units.