I always had issues when I tried to DM that I never really could plan right. The one real issue is the same Log has, is why are the players there, and for me, why the hell they should go do whatever quest.
The game I am in now is running good, well I'm dead in it, but good.
When I DM, I tend to fly by the seat of my pants, so to speak. I come up with NPCs that can easily be thrown together into groups for encounters, and a few boss-types, but nothing really set in stone. Sure, I have a general idea of where I want the campaign to go, but I have lots of different ways to get there, and 3 or 4, maybe even 5 possible endings already figured out. Which ending they get is entirely up to my group. This makes it more interesting for everyone, including me, because even I often don't really know what's going to happen next.
The DnD group I'm in has 2 long-running campaigns (for now, all 3.5) that we switch off between to give DMs a break. Plus, both the DMs have different DMing styles. I run a war campaign, but keep the storyline loose to make the player's have a bigger role in writting the story. Another DM is running a campaign that's actually based off of a book they're writting, and...well she can be a bit aggravating, because she'll randomly change rules to force us to do certain things, and can be so OCD about the story that it can suck all the fun out of playing. Generally it's fun, but on at least one occassion, she was having a bad day or something, and pissed off everyone so much that almost everyone walked way from the table. It doesn't help that that DM doesn't really know the rules all that well, and 2 of us players (one of which is me), pretty much has them memorized. Don't get me wrong, I'm all for houserulling things. I do that myself quite often in my campaign to help get the 'feel' I'm after. But she doesn't stick to her houserules. They'll apply one minute, but not another. Then they'll apply again. It probably doesn't help that the DM is fairly OCD (litterally). So much so that she even tries to tell us how to play our own characters. (example: I was playing a Evoker Wizard in this campaign, and we were exploring a seemingly-deserted village. I looked in a room, but did not go in. I cast Detect Magic on myself, saw that there was some sort of magical creature waiting in ambush, so I simply left...she tried to tell me that I would investigate the spider...). Basicly...she's a pretty crappy DM, but her campaign is nearly done (we've taken a break from it, because I had to stop playing for a while to focus on school, and I'm one of the main damage dealers), so we're all trying to be nice and will finish it out once I have the time/money for it.
Also in a 4e campaign that meats twice a month, with a few of the people I play 3.5 with. While I miss the customization of 3.5, I gotta admit that overall, I prefer it.
While I'm taking a break from the main group, they're experaminting with different systems (I know Paranoid, Star Wars, and Marvel. Maybe some others), and based on that, we'll be picking up long-ish term campaigns from one or more of those.