I don't think it's quite an emotional-expression thing, it's more that while Ten can be psychotic, his rage is still that of a kitten--he's all growling and trying to make the bigger animals believe he's serious, but he still ends up chasing his tail. Ten is truly more childlike. Eleven doesn't have to go crazy emotional on the bad guys because he's crazy emotional all the time. The lack of silliness is what's scary. (Finished in comment.)
