One thing to keep in mind is that the entire Sunwell patch was a surprise. The idea of 2-year expansions with 4 content patches hadn't been established yet, so no one ever really gave any thought to what a raid after Black Temple would be like. This made the whole patch feel like a "bonus", which probably contributed to its positive reception.
The daily hub was good. It was dense and everyone was doing it, which made it feel lively. The daily quests were fun enough, though not particularly innovative over what other BC daily hubs had.
Magister's Terrace was extremely difficult. Many of the BC Heroics were hard, and MGT topped them all. The PvP boss near the end (Priestess Delrissa) was a group killer; it was a common tactic to scout out what adds she spawned with at the start and, if you got a bad set, reload the instance. The dungeon was extremely CC demanding, which amplified the push for hybrid classes to get more forms of crowd control (though this ended up being far less important in WotLK).
I didn't do Sunwell when it was relevant, but I recall keeping tabs on the World First race. There was some saltiness over the gradual unlocking system (first 3 bosses unlocked the first week, then one additional each week after) as artificial gating that could invalidate World First race. But then guilds got to M'uru, who took 5 days to kill and filtered out a lot of guilds from the race.
Overall it was a really exciting patch, and I think the "surprise" factor was a huge part of that.