1. Same faction?: If your main is Horde or Alliance, then starting a toon on the opposite side means starting off penniless with no real means of affording good bags or gear for a while (I might be wrong on that count now given that the number of daily quests has resulted in considerable AH inflation, at least on my server).
2. Same server?: Same is true if you roll a toon on another server. My main is a Tauren Druid on a PvE server. The underperforming warrior is a Draenei Warrior on an RP server. Even with mining and skinning as professions, there is no way that the warrior can afford the things that my main can afford for her overfed, spoiled alts. The two warrior alts are roughly the same level right now and the Tauren's AH greens, plus enchants from a Mage alt leveling Enchanting, means her stats considerably outclass the Draenei's. Killing more quickly on top of being more likely to survive = easier to level.
3. Familiarity with the class/spec you're rolling: I respecced my Shaman from resto to enhancement for easier soloing and was suddenly jolted by an issue I never worried about on my Druid; if your weapon skill is 0, it's going to be a while before you can hit anything. Wow, man. Wow.
4. The Peeps: It's not as much fun to play an alt on a server where you don't know anybody and nobody knows you (I await the legion of comments that reasonably turn this around and say, "That's the reason for playing that alt, fool.").
5. Exploitation of said peeps: I don't think anyone can argue that getting run through dungeons for gear and drops is a less attractive option than braving an oft-empty LFG while leveling. Again, on another server you're on your own.
6. Class: Certain classes and specs tend to level faster than others, although obviously this doesn't apply to my two teeny warriorettes. However, my Dwarf Hunter has certainly leveled at (for me) breakneck speed compared to my Tauren Shaman. Let's face it; corpse-runs are often VERY long in old-world Azeroth and some classes are more likely to avoid them than others. That's a huge time- and irritation-saver right there. Yeah, yeah, I know, just Ankh, but let's compare the convenience of an hour's cooldown to a 30-second one?
7. The new factor: The quick little Tauren Warrior is also the first character I've ever taken Engineering on, and our resident engineer is correct; it's fun as hell. Or maybe you just take to another class more readily than you thought you would, with the same sense of delight over getting Vanish that you previously got for Cat Form.
8. Race: This could just be a highly idiosyncratic "thing" of mine, but the fastest alt I've ever had (the Tauren warrior) is a visual copy of my main with slightly different skin and horn colors.
9. How busy the original toon is: The main is frequently asked to tank things or assist other players. I have not yet been able to get away with pre-scheduling game time, as in "OK, I'm gonna finish dailies and get some farming done on the main, and after that I'll log to the alt." Never happens. All of my alts have wound up suffering from this, truth be told.
Some combination of these factors must be behind whatever spark keeps us playing a character that's not our main -- to the point where the toon might well become the main -- but maybe I'm just searching for a good reason to go back to observing an endless stream of "Your 2H mace skill has reached (preposterously low number)" on the shammy.