"seeing content" isn't fun. If people want to see content, they should try
http://www.youtube.com.
Progressing is fun. Beating content is fun. 5man-->10man/hard 5man-->Molten core --> ZG&BWL --> AQ20&AQ40-->Naxxramas
This was a fun progression path as a casual. It was never about gear, it was always about team efforts and putting your soul into killing the next evil badguy.
None of which require spending a lot of time.
Additionally, you learned the new neccesary skills to deal with the next tier by simply playing the previous tiers. It started simple and got more complex over time.
Also, simple and hard aren't mutually exclusive. If something is simple it only means that the learning curve is shorter, not that your marginals or initial requirements are very low.
None of the Molten Core fights were "easy" when first ran into with dungeon gear, they were however simple. And fun. And class mechanics were fun. BWL stepped up the complexity of encounters by adding more mechanics (but not more complex mechanics or class mechanics).
Because of MC experience, this made sense and added challenge. However the mechanics themselves were still simple. And class mechanics were still simple.
At the very peak of raiding, sunwell and naxxramas the mechanics were more and more complex but most of all you had smaller marginals. Class mechanics were still simple.
The only negative about vanilla& tbc class- and encounterdesign was lots of bugs and balance issues.
Combining class and encounter mechanics today you have so many small things to keep track of simultaneously. This isn't particularly hard since you're not even required to keep up with most of them, but it makes the actual important things much less satisfying, because they're just one thing out of a gazillion.
Edit: an example of current day really cool shit and what makes it less fun:
Heroic Al'akir.
Ret paladin. Saving people with instant word of glorys when being combo'd by storms and lightning. This is awesome in itself as it's situational and awesome utility.
However, I'll be interrupting my rotation to do this, the rotation that I do 95% of the encounter.
Additionally I have too many survival/save the day cooldowns. Lay on hands for every try, divine protection for every or every other wind burst, bubble, hand of freedom etc.
The cooldowns and "situational" spells themselves turn into a rotation. I'm now doing a dps rotation, a survival rotation, and a dps cooldown rotation.
The only truly situational judgement calls I make is when to delay my dps cooldowns/trinkets for maximum effect.
Not many spells are used reactively or by planning and coordinating. Ulduar(25H) was the godraid of reaction, planning, timing and coordination, with a perfect balance of required effort and time spent.