Not having DMed for quite some time, I'd say these are my strengths:
1. My love for DMing. Quite simply, I love every aspect of running a game. I love the system mastery and tactical combat. I love world creation and thinking up characters. The same goes for organizing the group and keeping things rolling. I love the plotting and improvising. Pretty much every DMing lets the overflowing imagination, that makes me the dreamer I am, run wild and then it lets me see what others make of the things I throw at their heads. This goes so far that I'm a terrible player, beacuse I get so damn itchy to be on the other side of the screen again.
2. As a DM, I'm ambitious. I want to improve with every session to provide my friends with the best game I possibly can. I constantly try new things and am always on the lookout for anything that could make me a better DM.
3. My NPCs are nice, but imho my villain gallery is awesome. I believe the baddies I create are original, believable and have stats to match. All my villains have their own modus operandi. Few things are greater than having the PC's itching to finally get that guy and I've seen it happen more than a few times.
I also used to believe that my challenging combats and stat builds made me an awesome DM. Sadly, I never had players as tactically minded as me, resulting in an overabundance of PC deaths and, yes, TPKs. Today I think this believe actually made me a worse DM, so in my next campaign I'll try for combats that are just as engaging, but with a far lowerkill factor, which will certainly be a challenge for me.