Insane crowds at launch, with a few small hiccups the first week after major problems during early access. After 1 month a decent chunk of the casual crowd will filter back to the MMO they abandoned to try the new hotness. After 2 months, many "hardcore players" will filter back to the MMO they abandoned siting lack of endgame stuff worth doing. The remaining 70ish percent of players, both casual and hardcore, will continue playing for 6 months to a year, with maybe a small percentage leaving a couple of weeks after the 2.1 patch.

Also some extremely easy way to level to 50 in an hour will be found, exploited, and fixed before you have a way to do it yourself.