Here are my guidelines for building an army:
Theme is always most important to me. I want every army I design to reflect the background for that race. I consider myself somewhat of a fluff Nazi; for example, I don't like the new CSM codex because it's very hard if not almost impossible to build a pure cult army. If you like Black Legion then count yourself as one of the lucky ones out there. That's just me though and I don't necessarily begrudge others for building armies that don't reflect their background material well. I know what you are thinking... Dante and Corbulo, everyone who plays BA are running that combination. While that is true I can say that as far as I know I was one of the first BA players to run them together. I'm not patting myself on the back, it wasn't my original idea but it was not commonly seen when I first started to use them together.
I want an army that is balanced and can win it's share of my games. I always put theme first BUT I always want to build an army that is good. You won't see me running a squad of scouts in my BA army... you won't see me running a pack of Flesh Hounds in my KDA. I want every unit I select to be a good choice period. I believe in synergy and that is where running a combination like Dante and Corbulo comes into my thought process. If a unit is almost too good not to take that in itself is not a good reason in my mind not to take it. I think some people intentionally handicap themselves - they don't want to be seen taking an army that could be perceived as WAAC. If you have a great theme how can anybody say your army is WAAC? I don't like a lot of spam as to me that hurts your theme. Theme and composition are two totally different concepts and some people don't understand the difference. Take my BA as an example. I have had some tell me they love the list as it is no longer frequently seen nor is it considered top tier anymore. I have also had people tell me that running two special characters is totally WAAC.
I always want to play unique armies that are not frequently seen. If the army happens to become popular later that is no fault of my own. I ran a completely unique 13th Company that was also a great army. I had a lot of people tell me to my face it was crap but I proved them wrong. My original BA army was unique - full Honorguard, two dreadnaughts in drop pods, terminators and scouts... Both the 13th Company and original BA armies can no longer be played now under 5th edition and are defunct. The beauty of running a unique army in tournaments is that your opponents will have problems figuring out how to react to it and that's a big advantage. It's great to build an army that no one ever expects to see; I think Marc is the best in this category.
I want to field beautiful armies period. This includes both painting and more importantly to me unique conversions.
5) The Fun Factor
If an army is not fun for me to play then I have no desire to build it. I can figure out by reading the codex if I would enjoy playing the army. I don't want to play any army unless it makes me feel inspired. I don't want to play a weak list to challenge myself. You are tying one arm behind your back if you take such an army to a competitive event such as Adepticon. I know it's all in the mind, what one person perceives as weak another might see as very strong. Many people have told me I need to stop playing MEQ. To me that statement in and of itself is not a good reason to not play MEQ. Two or three years ago I bought everything I needed to build a Feral Ork army. As soon as I heard rumors the list would be removed from GTs I immediately stopped working on the army and I'm very glad of that decision. I'm playing KDA now but if you stop to think about it the Bloodthirster and Blood Crushers both have 3+ armor saves plus my two Soul Grinders have front and side AV13 so for all practical purposes it has greater armor saves overall inherently built into it's design of mine. Sure the Bloodletters don't have that great a save but often a 5+ invulnerable save is better than a 3+ armor save plus if they go to ground in cover then it's just as good as Marines with the exception of most flamer template weapons. Probably my next army will not be MEQ and that is because I don't like the new SM codex and I finally looking forward to playing something else after all these years. Heh!
So those are my five criteria I use to select an army that I will actually build and play.