Tallifer
Hero
Themes.
Not every race, class, feat, spell, or other option fits in every world. If the setting is the Reformation Germanies then a Samurai is not going to be an option.
For material from books I reserve the right to use a line item veto, and I do not much me care if it is an official book or not.
The Auld Grump
I agree for the most part.
But then again I notice that players can come up with the most creative ideas when they have to try to make something strange fit into a world. I would love to see a good back story for a Samurai in Luther's Germany.