Try being a Warlock in Vanilla, and having to get your Doomguard. Ugg.
However, since it was optional, I have to say it’s not the Worst, it’s in the top 5,
Worst Zones IMO top 5:
5: Badlands (Vanilla). OP is correct.
4: Silithus (Vanilla), same reasons and less pretty than Badlands.
3: Vashj’ir (cataclysm) - This is the most divisive zone WOW has ever made. You either love it or hate it, personally I enjoyed it, but the VAST majority of players absolutely hated the zone. The hate comes from basically being a swimming zone. Turns out people really hated that gimmick.
2: Shadow Moon Valley (TBC) - Vomit Green, ugly, tough mobs, and required if you wanted Netherdrakes and the Black Temple. It would be the top place, but since it was optional content it doesn’t hit the number one spot.
1: Hellfire Peninsula (TBC) - It was ugly, everyone had to level there, on PVP Servers it was a place for level 70’s to fly in and gank lowbies, as there were choke points where all lowbies had no choice but to pass though. Hellfire made it so people didn’t want to level alts, once you had a 70 with flying, Hellfire was a fly over zone.
Edit: My bad, it was the Doomguard, bad memory at times. The Infernal Quest was on the other-side of the world in Fel Wood, and was actually a lot of fun. Doomguard quest was in badlands, and a massive pain in the Hellfire.