For me, it was the first time I set foot into Stormwind as a human paladin. I remember mirin' a max lvl on a Dreadsteed and thinking that's the coolest mount ever. I asked my friend if Warlock's can own one of those because I wanted to re-roll to have a demon pet. He told me it was a warlock exclusive mount. Instantly deleted my lvl 6 Paladin, and created a Warlock.
Back in Vanilla I rolled a Paladin - Holy Warrior - they said, Smite your enemies with your diving wrath - they said... ended up healbotting from BWL to Naxx.
So I decided to roll something on the red side too and I thought this way - it should be Damage Dealer only - no odd jobs (tanking/healing) - so it was either Rogue, Hunter, Mage or Warlock. Out of all of the options I picked up Warlock, due to the theme and that's how it happened.
At the start of the TBC I switched to Warlock full time until now.
Started playing at BC launch. Asked my brother what class I should play. Said if you like DoTs and being OP play a lock. Here I am 8 years later still doing it.
I liked the concept of demons and curses (killing someone slowly and painfully). I also liked the idea that the class was "evil" in the sense that while they align themselves with the H/A, they ultimately fight for themselves to become more powerful.
My love for magic. I have played one since vanilla (and a mage as well) and even though warlocks are fairly lackluster now, I still love mine too much to ever give it up.
I was failing at all magic classes, so I cheated by asking demons for help. It all went downhill after that, turns out you just can't back out of a demonic contract.
In vanilla i started of playing a warrior which eventually became my first 60, tanked alot of instances on the way, got defensive stance at lvl 50.
somewhere in between there i saw a warlock with a voidwalker, asked my friend what it was, he didnt know so he told me it was the warlocks father helping him.
For some reason i thought that was cool, so i created a warlock.
Started early TBC ,opened up the character screene and had a quick look trough all classes information and picked a warlock. Have been my main seens that point.
Made a priest in Vanilla, leveled to 60, did a single Molten Core raid and decided healing was boring. Made an alt warlock while considering giving up the game, and the moment I got Corruption and Curse of Agony something just clicked.... This was it, this was the class I should've leveled. A debuffer.
10 years in, the game is still a lot less fun (to me) as any class other than a Warlock, regardless of how different they are from back in the day.
Demonology. Fire and demons everywere, and semi-melee with Metamorph...it caters to my pyromania AND sadism at the same time, all the while looking awesome! Shame I figured out too late after getting too ingrained with my hunter whom I have a love/hate thing with atm...
Played a gnome warlock since vanilla i just loved how i saw a pvp video back when youtube was not the main source of wow vids but GOOGLE lol and I saw an undead warlock fear someone and had dots on him and drain life and got a few angry wispers from the player.
Was close to rerolling class the soulshard system summoning pets wasting shards on summons + healthstone everything required a soulshard had to use 1-2soul pouch and barely had backpack slots for my personal raiding consumables etc. ( lets not forget ur voidwalker could not fucking contain aggro even 1heal tick from health tunnel or weakest dot siphon life would over aggro it! )
Was making bank gold $$ farming the eye of shadow in wintergrasp decided to get my epic mount quest and boy it was epic the questline.
but I loved pvp on warlocks FEAR , Death Coil , howl of terror lol this class was at the peak in tbc <3
SOLO ASPECT
During CATA i checked youtube videos on warlock solo x and enjoyed how great warlocks could solo raid bosses and even got my ashes of alar during mid cata.
TBC - WOD warlocks was fine in dps wise loved the opness of affli locks in SOO
My Warlock was my first toon in WoW, I believe I chose it because I came from Diablo 2 where the Necromancer was my favorite class so I figured the lock would be most similar to the necro.
I remember leveling it to 28 before even realizing what talent points were (this was back in 2006), I was so angry at the time but obviously it's something I laugh about now.
For me, I thought it would be cool to blow through Auberdine as a horde warlock and throw a dot on an unsuspecting ally and just run away while they die slowly knowing there was nothing they could do about their impending doom. That was many years ago and I still enjoy them for their DoT mechanics.
Edit:also I thought the felhunter was the coolest pet ever, and I loved doing the quests to obtain them. The Dreadsteed quest is my favorite memory of wow.