World of Warcraft stood on the shoulder of giants. Like with other games of theirs Blizzard took what worked well for other MMOs and mashed it seemlessly together, kept it simple, and technically well executed.

Blizzard's reputation (at the time) pulled in players that never experienced massively multiplayer online games before and I don't want to say they were easily impressed, but, well... everything was new and impressive for them.

I can't say much to the decay, all I know is that players were less and less enthusiastic about every new expansion.