As the US has found out upon the initial update to of Blizzard's Hearthstone,'The Mean Streets of Gadgetzan', there has been a major issue with the new cards.
If you pre-purchased MSOG packs, DO NOT OPEN THEM until Blizzard notifies that they have fixed an issue causing packs to provide unusually high chances of pulling a 'Tri-Class' card - a common or rare card type available exclusively for three cards. This dilutes your chances of finding a much rarer card significantly, and you will end up with a tonne of uncommon 'Tri-Class' cards if you make the same mistake as I did and opened all 50 of your pre-order packs at once.
Sure, this isn't going to be relevant to a lot of you, but I figure if it saves one person's purchase then that's something.
I want to thank you for posting this :)