I am pretty happy that I haven't had fundamental problems with my players yet. But I am in a steady group nearly since I began playing. Most of the other players are older and more experienced in RPG than me, so my play style couldn't really differ so much from theirs that we could "collide".
Our group also has the rule that every player has to be GM or DM, and every week we switch the GM/DM. That has the severe disadvantage and destroying a lot of plot cohesion, since players tend forget what happened in the past session of the DMs campaign, since it was 4-5 weeks ago and each week, another story was followed.
But it has the distinct advantage of giving everyone enough time to prepare an adventure and everyone having a lot of time to play a character. None of our group members really has enough free time to be a steady DM.
Reasons why I personally stopped Campaigns:
- Lack of Stamina to bring a Campaign that I had planned to an end. I think it would have worked out, because it's not exactly difficult to steer our group in the right direction, as long as the adventures are entertaining enough. But at some point, I just didn't enjoy having to play through the things to the end. Maybe this is in part due to the complexity of the story and the rules, at which point fleshing out the adventures becomes too time-consuming, and no longer seems worth the reward.
- New Settings or Rule Systems that I want to try out.
I think it's usually a combination of both that I stop.
My first big Campaign was a Dragonstar-based campaign in a word I had in some shape or form in my mind even before I ever played Role Playing Games (but there are several variations of that world and its storyline in my mind). The campaign ended at a high level, with still a few plots in my mind I wanted to finish, but somehow couldn't bring myself to do it.
I then stopped to run an Arcana Unearthed/Evolved campaign. The campaign went also into high levels, and I had a pretty clear storyline (involving Virdella Thresham and her attempts to destroy the plane of the Diamond Throne to get more power). Just like with Dragonstar, I couldn't bring myself to really complete the story. Part of the reason here might also have been that I had aimed to end the Camapign Arc at a higher level, and thus meant I had to add more adventures in between.
My next Campaign was a D&D campaign with a homebrew setting. I had some ideas for a further adventure, but I became less interested in it. In addition, I finished my Diploma Thesis and took a full-time job, so I had less time to continue creating adventures.
So I picked up Iron Heroes and began to run "Dark Harbor". I am not certain I will continue beyond that adventure setting, since both Starwars Saga Edition and MonteCooks World of Darkness look tempting to try out.
So, I guess my best advice to myself would be to reduce the complexity and the time frame of the plot-lines I want to use. "Mini-Campaigns" instead of a plot that becomes more and more of a burden. (Maybe "Points of Lights" and/or the Pathfinder approach is the best way to go. Start small, with a lot of blank space and "Here be Dragons", and see how it goes...)