The technical aspects.

I'm a little bit of a mix... But I like efficiency.

I start out with a basic idea--for example, I wanted Minimate (AR/fire blaster) to be an AoE slaughterhouse, and I wanted Frozen Cap (electric/electric blaster) to be a chaotic irony machine. I use whatever relatively cheap set IOs I can to make them better most of the time--Minimate has about a 40% global recharge bonus so the AoEs do more damage in the same amount of time, and Frozen Cap has somewhere around 15 damage procs for the sake of chaos.

Those probably aren't my best examples, but I haven't gotten most of the more interesting ones to 50 yet. :P (Hitless = dark/force field defender for the sake of preventing enemies from hitting allies; Har D = hardy dominator with good mez, regeneration, and defense; Unresistable = psionic/electrical blaster, which are pretty much mutually exclusively resisted damage types)

I've done quite a bit of testing and number crunching in other ways, too. I discovered that resistance debuffs add up based on your base damage, rather than your enhanced or enhanced-and-buffed damage... Found useful holes in various maps, made lists of enemies with negative damage resistances, and I took notice when my DoT stopped doing damage if I was defeated and then resumed if I rezzed before it wore off. (They've fixed that one!) I also like to play with slash commands... /enter_base_from_sgid was one I, uh, "mentioned." :P

Pretty much, I manipulate what I'm given until it's about as easy as I can make it. Games have always been about fun, and I'm the kind of person who would enjoy a game just as much, if not more, if I had an invincibility cheat. (Actually, I'd like to start at level 50 so I don't get bored with my characters by the time I can set IO them.) :P

IOs and more IOs.

I plan each build out with sets in Mid's, level them while speed bumping for Accolade requirements, slot them out with IOs then start over again.

Occasionally I'll get a humourous/RP concept and it'll actually turn out to be quite fun (my Trick Arrow defender, Warshade, Kat/Regen Scrap). Mostly I just play to have fun, though the PvP debil is always at my shoulder, so that's a concern as well.

Concept first, then optimize.

I get ideas for a character, try to figure out what makes them tick and what my purpose with that character is. Then I optimize for that purpose. I can't play a character optimized just for numbers, I get bored. But simply putting a few sentences worth of concept behind it and it all gets much clearer. The sorceress specializing in barrier magic because she wants to protect the defenseless, the professional ninja who never breaks a contract and dresses in white, the electricity slinging mutant daredevil, the woman inheriting the trick archery powers of a hero slain in the Rikti War and so on. For some reason, most of them are female.

When I got that in mind, then I optimize. I always have a few character concepts to spare in my mind, so making a new character is easy, but I can't simply pick two powersets because they're the best. A Fire/Kin Controller would bore me without a really good reason that it's both Fire and Kinetics. Fire/Thermal is a much tighter concept in my mind. I have a Fire/Emp angel somewhere, although she'd get rolled Fire/Thermal (colored white in I16) if I were to make her now.

I don't plan out IOs too much. When the character matures, it becomes obvious what bonuses would be nice, so I go for them. My Ninja/Ninja Stalker went for Damage boosts, for instance, but it's not planned out in advance. I do buy sets when the characters get high level, because... what else do I spend my inf on? But until 35+, it's generics all the way.