There will be too many day 1 players that it will make things unplayable. The starter zones where not designed to handle a large number of players at once. Once you get further into the game that changes, the zones are larger, you can do instanced content and there are different ways and places for you to level.
From my understanding they will only have sharding for a short time and I was told it will only be in the early zones this is perfectly acceptable. Without it I fear no one would get anything done.
![:space_invader:](https://d38bqls1q93fod.cloudfront.net/en/wow/images/emoji/twitter/space_invader.png?v=6" ":space_invader:")