The most impressive aspect of WoW to me was just how well the game ran on commodity hardware at the time. EQ2 came out the same month and ran very poorly on top-of-the-line PCs. Vanguard beta hit a year later and it was nearly unplayable on anything but the most cutting edge of systems. I really think Blizzard's focus on performance optimization and UX had a very significant contribution to the success of WoW and widening the playerbase.