You touched on something here, Weem. I knew this guy who took acting lessons just so he could be a better role player and work on his spontaneous GMing and not doing "oh's" and "ums" was amazing. He did multiple voices, styles, etc.
This is a really good point, because I also sat on the end of a table of a guy who didn't do any role playing and just stated "you do this" or "you do that" and that was fairly boring.
My brother has quite a bit of experience as a DM now, but he was always a TON of fun to have as one even when he was new to it - much of that had to do with his amazing wit and improv skills - he did a lot of acting and theater and it just completely made the game. Each NPC you talked to felt like a completely real and unique person and it really draws you in.
The rest of the stuff like understanding the rules/mechanics come easier (imo) than being able to articulate RP scenes (in a very compelling manner), and to portray NPC personalities that are unique to each one you encounter, etc.
Part of that can be learned and improved, but a lot comes from talent I think, and he has it. I have been DM-ing a long time (muuuch longer than him) and it's still something I work very hard at - it just happens to come more naturally to him.