1. His mustache.
No seriously. Look at this thing! It's ridiculously awesome!
2. Quotes like this:
"Clothes make the man. Naked people have little or no influence on society."
"The man who doesn't read good books has no advantage over the man who can't read them."
3. His ability to comment on various social injustices while still making us laugh.
4. He wrote his autobiography, but insisted that it not be published until he was "dead, unaware and indifferent". He died in 1910. The University of California Press hung onto his complete memoirs all this time and will publish the first volume in November. 100 years later? Yeah. That sounds about right.