I'm that DM that has storylines that take place over 20 years of gaming, multiple editions, and many different groups.
I'm that DM that uses time travel to soft reboot the campaign setting so that I can explore new ways for it to unfold with new players. I'm on instance 4 or 5 (depending upon whether you count a fizzled campaign) of the 'first adventure' in the primary setting. Each time it has run it has been very different depending upon the decisions of the PCs, the edition rules, and the tweaks I've made to the setting.
I'm that DM that considers the world I run locked in place once I make the decision to add something to it. Even if the PCs have not encountered it, I do not change it. It is there, and I figure out how it interacts with the rest of the world and I add it to the checklist of things I need to update as the calendar moves forward.
I'm that DM that uses about 50% homebrew spells, monsters, and magic items.
I'm that DM that plays the long con within a campaign - with PCs discovering that something from their backstory or something they encountered at level 1 might be a major element in the campaign's final stages.
I'm that DM that wants every PC to have a starring role in the campaign at differnt points in time. The PCs are not playing in my campaign. We're building theirs.
I'm that DM that puts old PCs from prior campaigns into his future campaigns as NPCs.