Not my first but probably my most successful D&D campaign, Planescapin began back in 2013 and ran until July 2016. It finished successfully, albeit slightly rushed due to growing player fatigue. For a Planescape campaign it was surprisingly Prime focused, through my fault rather than intention. The group never really got involved in faction politics or the great conflicts of the setting, with the planes becoming exotic locales for them to visit in between wrestling with plot. What follows is my own retrospective of what happened, what worked and what didn't.
A group of low-level Prime PCs would gain access to a planeshifting castle, allowing them to instantly start adventuring on the planes and have a route back home. Plot hooks were devised to interact both ways, with their Prime Material having links to the planes in order to allow the PCs to explore their Prime while keeping in the Planescape theme. The main plot of a dragon apocalypse was set up as a vague background story, since I hadn't decided on the details of it at that point.
What the Players made:
The starting party was an eclectic bunch, held together tenuously by the 'inheritance' of the castle. The roster also changed considerably with only three of the original starting PCs making it to the finish line. The party leader and face was settled early on though, providing a stable core around which the infighting and solo adventures could take place.
The PCs had a lot of early extraplanar adventures but slowly became more embroiled in events on their home plane. While I did lead them that way, intending for it to break out into planar politics it never worked out that way. They came to see the problems on their home plane as paramount and they spent a lot of time struggling against the corruption of society rather than any concrete foe. Looking back now it's easy to see the gaps I should have filled in to point them to their extraplanar threats, however at the time I was worried about info overload, which ironically happened anyway. The game was very open and the players did sometimes use that to detour into pursuing their own interests. However the openness meant sacrificing the depth of the plot for breadth. I'd consider too much freedom better than too little but I have yet to achieve the balance.
Comments from the players:
Compression: The NPCs that were there were great but less NPCs would allow for deeper interaction with those present.
Narrative Imperative: A lot of the later plot hooks left a feeling of pressure, with little time to faff about.
Vagueness: Clues were often vague with little to indicate what to do next. This led to incidents of murderhoboism in an attempt to forward the plot
Crafting: Crafting occupied a lot of time and energy of the characters, and led to them undergearing as they saved up to build gear themselves.
Railroading: In some instances the characters would have been happy with just being told what to do next
In all we played a combat-light, political and dungeon crawling game with an open world and Planescape elements. It was enjoyable and we all invested a fair bit into building both the world and the characters. The open game was good but could have done with just more GM steering at times.