I remember the first time I ran into a warlock in vanilla WoW. We were both around level 40, and I met him in an Uldaman run. I didn't even know the class existed until he responded to my advertisement in trade chat. During the dungeon, he was constantly Fearing mobs into other groups, accidentally tab-targeting and tossing DoTs on patrols, and letting his pet steal aggro from the tank. My first impression of warlocks was a negative one, and it took a long time for that prejudice to subside.
Most players' first experience with a rogue will be on the receiving end of our daggers. Whether it's in a battleground or in the open world of a PvP server, rogues are killing thousands of players per hour. If a rogue gets the opener on an unsuspecting player, it's not a pretty sight. The target will be ambushed from nowhere, stunned, slowed, interrupted, poisoned, and bled to death. Being killed by a rogue isn't a fun experience. You're hit with the shock of surprise, denied control of your character, and incredibly restricted. It's easy to see how a player's first experience with a rogue could leave a bad taste in their mouth.