Mostly play, sometimes GM. I just enjoy playing more. And frankly, I find it hard to be DM. I don't mind spending hours building classed NPCs for the party to kill, I actually sort of enjoy that. But I struggle with thinking up a plot/story, can't name things easily, and really really hate mapping. Thinking up specfic rooms or environments to have a fight in with difficult terrain, high ground, enemies coming from multiple places, etc... is fun. Putting together a dungeon map or world map, or even the layout of a town? Hate it. I don't want to run mindless dungeon crawl hack and slashes, and I also don't like using adventure paths much. So...I end up seldom DMing.
Sounds like you have mastered the hardest thing, which is making a villain or monster that is challenging but not a TPK, so I would gladly play at your table.
I am like you in that the long term plots and storylines that are fun and interesting are really hard. But, I don't know anyone, even professional writers, that this comes easy to so I try to do the best I can and come up with something. I do have a secret resource though :
The Thirty-Six Dramatic Situations - Wikipedia, the free encyclopedia
It has some good plot ideas, theoretically ALL plot ideas.
For names, when I am tired, there are a lot of Bobs, Steves, and Joes that end up in my games, but when I am on the ball I will try something like:
Character Name Generator
to generate names for me.
As for maps, I couldn't make a decent map to save my life. So I steal, like crazy. If I find a map for a town, I use that as my town, just change the people that are in it. So I have used the city state of the invincible overlord many a time. Hommlet shows up all the time, etc. I have been gaming since '77 so I have built up quite the collection that I can scan in and use.
For dungeons I do the same. I take a dungeon someone else made, repopulate it with my monsters, and then throw it into my game, presto, professional looking map. I can upload a jpg of it to image shack and link that to OpenRPG, do a little shading and the game is on.
Trust me, I am noone's idea of a perfect DM, I am a much better player, but the game must go on. So when noone else will step up, I try to take my turn at it. It is work for me, not like back breaking ditch digging work, but still work to think of and write decent adventures. I don't know many, though I do know some, that it comes easy to, so don't worry that you have to struggle at times. Most of us do. If your players are DMs/GMs as well then they will certainly appreciate the break and will be supportive.