Often I find myself wonderin why I chose the armylists that in do. Typically you won't find me fielding the common, the popular or even (to my usual dismay) the effective. When I read through a codex I take in the fluff and the rules and weigh them equally. This leads me to my own opinion on a unit not solely based on fluff or stats. I know most players chose units that are pure power. The unit or model is chosen because of it's value in the mechanics of the game.
Now when I consider what is a "good" and a "bad" unit I find my opinion crafted at the moment I read the codex. I lean towards units that "feel" or sound cool. From the outside this might appear to be along the lines of when we were kids and chose models because they looked cool. Let me explain how it's different. In the Chaos codex I like Greater Daemons. I don't recal the last CSM player that I've seen w/a Greater Daemon. Sure you see Daemon Princes but the GD is just not seen as good. I've heard that people don't use it because it costs a champion model. I don't see this as a detriment or an increase in points or even a waste of a champion. I think this is awesome! If you have 4 units of troops roaming across the board and a Monstrous Creature bursts out of it's ranks to charge the turn in comes in? Sign me up!
Another example would be the Pyrovore in the Tyranid codex. No one uses them. I have never in my gaming experience seen any Tyranid player use them. I love Pyrovores they have Heavy Flamers an Power weapon attacks. On top of that they blow up. They blow up! It's rare but I played a game where Pedro Cantor blew himself up by powerfisting a Pyrovore. That's 100% awesome! Why wouldn't you want stuff like that to happen? Admittedly I'm the craY guy who cheers on the opponent and wants him to blow up my vehicles. I just enjoy the odd rules and the crazy possibilities in a game where you generally see the same thing over and over again.
This is what usually drives me to determine what is goods and what is bad.