Utterly without doubt, with the announcement of the expansion in a couple of days time, there will be the announcement of a preorder.
With that preorder will almost definitely come new, immediately available digital stuff available in game, particularly in the form of mounts, minions and potentially armor.
There are many, MANY people who will use the logic that since they are definitely buying it anyway, they might as well buy it now and have the stuff now.
If this is you, rethink your position. The preorder content will still be there months from now. The behavior of the consumers will inform the development of the expansion for years to come.
Demand more of Blizzard than we have in the past. Demand physical, concrete evidence of quality production before you give them your money.
There will be plenty of people who will buy the preorder two days from now and nothing in the universe will stop them. That's their prerogative; they aren't the target of this post. If you can put off the short term gain of mount number 358 for a few months, or even weeks, you stand to gain for years to come.
There is a reason they do the preorders in the fashion they do; they predate on feelings of FOMO. You won't miss out; those things will be there for months and months. Please hold of, so that we can demand better.
I'm not saying don't buy the expansion; I'm not even saying don't preorder it. Heck, I'm not even saying don't preorder it soon. Just don't do it this week, or possibly even this month. Wait for more information, for more evidence you are buying a well made product.