Emirikol said:A character may only shape change into something they have actually seen before. Drawings in books and 'concepts' do not work.
This can both be a good rule and a bad one. On one hand it limits what the pc can use, usually down to acceptable limits.
On the other hand it is a problem when the dm says, 'well, I would like to use these guys that I made up/saw in magazine x/have in the module.. but once the pc sees them he'll be able to change into them' This either causes the dm to have to change a lot of things, or just ignore the idea.
On the same line as that however, if the dm never uses anything that is 'useful' in some circumstances the spell is effectively wasted. If everything you could ever want to change into is either not a valid creature from type, too big/small, too many hd, or even the dm saying no to the change what is the point? It would be just plain better to say, 'at levels x and above it can do these abilities, at levels y and above it can do these abilities, etc' or have the dm come up with a suitably varied list of things that can be changed into (something like the new command spell).