I believe the idea of the expansion was an incredible idea. New races and a "new" class newly allied with each side. The 5man dungeons with the idea of them turning into heroics is a great idea, but poorly implemented. The only real achievable loot worth doing the heroics is the final boss. This piece of gear is supposed to make the whole waist of time worth while. I understand that OOoOOooOO its an epic, but who really cares about the fact that its epic when other blues are equally as good situationally? The progression up to 70 is also an amazing accomplishment implemented with ease. I find all of this to be absolutely incredible about the expansion.
The Biggest Mistake
Having Karazhan (A 10 man 7 day reset instance) Be the stepping stone into 25 man raids. I feel that this has hurt the game even more than anyone could possibly imagine. The people who complained o, I can't get uber leet gear still won't because they lack the skill to raid. Those medium sized guilds that created with alliances with other guilds will prosper much more than any other guild out of this change. The 40 man established raid content was set in stone for 2 years. Guilds built up to 40 people obviously a core of strong gamers, but those who could fill out the ranks and do half decent allowing for good progression.
The guild I was in was a good portion through or finished with most wings of naxx besides ghoul. The first people to 70 made their way through Karazhan downing attume first try and a laughable joke, but the lack or need of specific classes prevented any further progress first night. As the other classes leveled up and got into Karazhan Rogues became the first to completely get kicked out later to have some stuff changed and allowing 1 or so to be let back in. None the less the balance from a 40 man to a 10 man is completely different, and requires the shaving of guilds completely.
I know many guilds will send multiple groups through Kara, and run through gruul's lair with ease, but none the less I feel that the biggest mistake with xpac is creating this complete bottle neck at Karazhan causing in some cases the complete destruction of guilds and everything that has been built up.
I quit due to this issue, and I may come back next year when another xpac comes out to see how it is, but the Burning Crusade seriously needs to look over how they treat timing for raids. 20 mans were progression to 40 mans at 3 day timers. Now a 10 man is a progression to a 25 man at a 7 day timer. Completely uncalled for in my opinion, and I think has lead to the quitting of a lot of people. Maybe not, but I know 10 people in my circle of people I played with who uninstalled their copy 2 or so months after BC release due to it.
If ya feel the need to flame this opinion due to your own e-peen stroking go for it, but this is just how I feel.