I am an altoholic. I had a main in vanilla (warrior), because raiding on more characters was a mess back then.
Then I migrated across the Atlantic just before TBC (no fancy server transfer there, it is the big old re-roll style. Even went all out and faction changed too). This means I don't have all my exalted vanilla reps, and since it was before achievements and transmog was added, there were little in terms of doing old content. I just sort of powerleveled a new warrior and took it from there.
I also leveled up a hunter for farming, ended up doing a ton of PvP on this one before TBC hit (there was that crazy christmas period where they changed the honor earning and everyone could get the HWL gear by playing enough to get the honor to buy it). In TBC I also had a shadow priest for arena, just to try that out.
Lich King was where it went completely bonkers. I had to try the DK (being a passionate plate player), so I maxed one and started raiding on that, then after a month or so, I started missing my shield and leveled my warrior again. Then he took over my main raid group's spot. But it does not end there. The main raid group (bunch of friends) had pretty slow progress, so we made a more serious alt-raid on alternate days. I brought a hunter there and this raid turned out to progress faster than the "main" raid. At the same time I played weekly with another group of friends leveling slowly, so I had a pally and a mage for that group - which eventually got to max level as well.
Then I took a long break, skipped cata, and came back for 5.4.
Leveled up my DK, then started missing the shield, so leveled the warrior. Then found out that I did not want to deal with guilds, so to optimize my LFG/LFR playtime I decided to level up a druid to queue tank+healer. The DK, warrior, and druid now all have 540+ gear and sort of "done" in terms of non-guild playing. I have MoP reps scattered between them, depending on what I needed for tradeskills, so none are really that far ahead of the others. To round up the altoholic addiction, I actually enjoy tradeskills a lot, so I had to have my JC+Enchanting shammy leveled up as well, but right now he is the only one of my 90's that is just sitting in TI gear. Last fling is a warlock I leveled up because I wanted to try the goblin starter area and the warlock green fire quest. Having a small army of 90s, all the tradeskills and good experience in powerleveling means that it took less than a week to get him through SoO. The gear is a little lacking due to RNG and the fact that I spent most of the week farming for a sealed tome. Finally got it last night, so now I am off to the do the quest and who knows what happens when I finish that. I still have my hunter sitting at 83 waiting to get leveled, and a pally+mage that might get some playtime as well. Or I would level my priest since I havent played that since TBC.
Likely by the time WoD hits, I will have all classes at 90, waiting to see which I would play to 100 first.
One thing is certain though, my boost will be used on a rogue. I had one at 60 in vanilla and I never liked the class (but it was by far the best miner alt on a PvP server). So the plan is to boost a rogue to 90, level lockpicking and use it to open lockboxes, so I do not need to use ghost iron for that.
And then there is the whole alliance aspect... still haven't played worgen, and haven't played Alliance since vanilla.
The biggest change in the game in terms of main vs alts is the sharing of achievements between characters and BoA items. For altoholics those are just awesome. The commendations for reps are great too. And I would love to see something that could let me farm gear for my characters on a different character (like a way to share/transfer valor).
But to answer the initial question. My "main" is the one where I currently have a target set. It is likely to change as soon as I achieve it, but while a goal is active, it will be the character I log in when I have time to play.