[quote="Julie Skywalker"]Mufasa was old. OLD. He was visibly old and he was a monarch.[/quote]
I personally don`t think Mufasa was THAT old. He seems to be a mature male certainly, but still in his prime when Simba is a cub.
Anyway, like Julie Skywalker said, Mufasa had a huge responsibility resting on his shoulders, but we did see Mufasas playful side more than once. He seemed to have a good sense of humour, was kind-hearted as well as stern and powerful. I think the balance between his different roles as king and father was shown clearly.
In TLK, the young adult Simba has grown up with the Hakuna Matata philosophy, he hasn`t been needing to take any responsibility and so if you compare him and Mufasa, the older lion may come across as too harsh and serious, but I personally feel that comparison is unfair to Mufasa.