The texture pack that comes with it is unusable as is (really out-of-date and not user friendly), but we are waiting for permission from its creator that we can use a version which Obsi has updated for us (thanks Obsi!). We will make the original texture pack available regardless, but you may find it is better to play without it.
As I haven't yet heard back from him, I don't think I'll be given permission to redistribute it before the event
The only major difference is that the bow and arrows become a wand that shoots stars, which is kinda cute... and the glass and other block textures are more in keeping with the theme than the default pack..... If you don't use it, a few things will look a bit weird...and Diagon alley will look really bizarre in any other texture pack
I personally find the default one unusable. There is no crosshair, and in the hotbar, there is nothing to show which item is selected. Making it very difficult to use in battle situations. Also for some reason it has a ghostly wand, map and diamond built into the hotbar (presumably suggesting you place those items in those slots), which is a bit confusing if you don't place those items there. Plus the beacons and other new things aren't in that pack.
I "fixed" all that, but I need the maker's permission to pass it on to anyone... so I've asked, but not had a reply yet.
So any sort of medieval type pack would probably be a good substitute though.