Vanilla really wasn't really that great.

Vanilla had it's bright spots and great memories for sure. Striving to get that helm from Scholomance (grats to those who got it), downing those big bosses that were hugely intimating (especially for MMO n00bs), and watching people die in circles for hours in Hillsbrad. These were some grand moments and they were undoubtedly fun, but they were also kind of "all or nothing" things that were very frustrating for me and my guildies when they didn't happen as we wanted them to. (/shakes fist at Scholo)

I have found that I played in a bit of a different way in TBC and WotLK by either raiding a lot or playing a lot of alts (sometimes both but not consistently). What is fun for me in the game has changed but has allowed more things to be achievable and therefore I ended up playing more as I was getting more things done. Yes, I also like achievements and have checked off a lot of things from that list.

Q u o t e:

So basically you like how the game has gotten easier and more casual.

I like how it has provided me with more things to do. It is easier to play my alts as well, sure, but there has also been more reason (achievements) to push on progression and I would say the progression is harder at the top end as Vanilla wasn't too bad when you minimized the amount of piggy back rides.