From what I understand, most of the effort went into coding Vanilla and WotLK servers. To put things into perspective, only one server that I know of (Gamer's District) has the whole Wrathgate/Siege of Undercity questline properly working, and it's been that way for less than a month now. Too bad GD is a 2+year old fun-server. The fact that each individual p. server's team won't 'share' their own code/work with the rest of the developers also greatly 'hindered' any form of 'progress' in a timely manner, where private servers are concerned.
Hell, I've yet to see Nostalrius release their code as of yet, they're probably much busier blowing Blizzard's WoW devs for a chance to work at them, when Corecraft's PTR is 10x the server Nostalrius was (I'm exaggerating, of course, I just don't get why so many people are ignorantly praising Nost as the p.server haven when there were still so many bugs present up until the server's demise, not to mention the abysmal actual gaming experience due to the ridiculously small LoS, people were yelling for a 3rd server daily whereas the devs were to stubborn to accept any money or try to look for new GMs).
It just goes to show just how stubborn most private server 'developers' are, and how desperate most fans of the older WoW are, I guess.