I took holidays to enjoy the ride to 80. My feedback:
I really loved
i) the environments
ii) the variety of quests and
iii) the lore
Overall I give the experience top marks. I had great fun.

Two main observations:
i) Difficulty - There were a number of aspects that I thought were too easy including a number of group quests and most of the normal dungeons. With every tanking class being able to AOE tank normal dungeons all you need is a reasonable tank and healer and they become very easy. However I've done a number of heroics and I found some of those to be a real challenge. I also did 2 wings of NAXX with old friends mostly in not the best quest blues and it did seem too easy. I like that the wings are instanced and I hope this means my casual guild with their once a week group play can take on NAXX without having to devote extra hours in one week to do a full clear.

ii) The WoW community. When I first started there was a lot of courtesy buffing and friendship. Now the community is much more solo-oriented and less 'community'oriented' and general chat is so often something to cringe about. Sadly I think some aspects of game design foster this.

Two suggestions:
a) Have 2 modes of group quests - 'soloable' and 'hard difficulty' - with different rewards -I realize some people like to solo and sometimes getting a group (particularly in a few months time) isnt easy. The way quests chain it is necessary to do every quest to open up other quests.
b) Look at some form of public questing system (hint: that other MMO) for some quests to foster groups and friendship

Quite clearly from reading this forum the current 'hot' topic is how much WoTLK caters to the 'casual'. I don't think the casual - hardcore binary opposite distinction is useful. I did a survey of 3 'hardcore' and 3 'casual' guilds last year and looked at hours spent playing and playstyle. The stereotypes simply do not fit the labels. There is a significant proportion of self-defined hardcore players who log on to raid and farm materials, who have careers and familes and restricted playing hours. Equally there's a large proportion of self-defined casual players who play a huge number of hours per week (for me a staggering number) who either don't like raids or (and this is my opinion - not what they say) cannot access better guilds because they lack either the playing skills or online social skills. The hardcore basement dwelling too many hours per week 'hardcore' and the casual 'has a life', restricted hours stereotypes simply do not accord with what I observed.

What was commonly expressed amongst most players was:
i) they wanted to see all the lore (including the ultimate bosses like arthas) (interestingly this desire was less amongst long play hour casual players)
ii) by and large they wanted some parts of WoW to be challenging - although for most casual 'restricted hours' players challenging meant not impossible and they wanted to be provided with gearing up pathways that enabled them to take on instances some time after the hardcore players had finished them.

Two suggestions arise from this
i) implement spectator mode
ii) dont label 10-mans normal and 25-mans heroic - instead have heroic versions of both with slightly better loot and make this the real challenge for the 'hardcore' and the expert restricted hours casual players. (And to make design easier heroic could just be as simple as 8-man and 20-man)

Thanks for reading.