generally server stability isn't a problem most of the time, there's two things/weak points they'd be worried about:

1: what happens when everyone clusters into one zone - you can see hints of it in Dalaran in WoW (only hints, because they're managed pretty well), where the server is dealing with so many connections on the one node. This causes major lag for anyone in zone, and also manifests as loot not appearing, mobs teleporting, etc. Any expansion release this is a likely occurance, as everyone floods into the new content.

2: the download servers - if everyone wants to download a patch at once, say the patch is 800mb - there's gonna be a point there where MANY MANY people are downloading it at once - meaning horribly slow download speeds, possible patcher timeouts, etc - all things likely to annoy players.