I'd like to add the following reasons why Ulduar is revered:
- It was large, detailed and highly varied in asthetics perhaps the most varied raid ever made
- It had the highest number of achievements to date, staggeringly many giving lots of replay value
- It had some of the best artwork ever seen on armours and weapons
- Algalon was exceptionally cool
- It offered some inspired hard modes
- Many of the fights were very fun, even Flame Leviathan (vehicle fight)