I was one of the goofballs that would play Rammus top, mostly as a counterpick vs an AD top like Yasuo or Renekton. Rammus' DBC made his farming very safe: turn it on, go in and last-hit some minions, back off. The design discouraged people from hitting me, and I didn't want to poke them or I'd overextend and run out of time on my DBC. If the jungler came, I could just powerball away. No real interaction in lane. After a few ranks of DBC and a thornmail, I was basically invincible to an AD top, and could farm entire waves by standing in front of their turret with DBC on. They'd have to hug turret, and if I snowballed, even that wouldn't help. Here's an example of my toxic play patterns. Note how lopsided it is even though I'm a level behind. The riven can't even interact with me by spamming laugh without getting punished for it.
The recently-added slow on DBC was brilliant because it had nearly no effect on jungle Rammus (who doesn't need to move to farm) but forced interactivity and windows of weakness with Top Rammus. I can no longer use DBC while safe behind minions, or it will run out by the time I get to the enemy minions; I must DBC closer to the enemy minions, giving the enemy a window to attack. I can't run away with DBC on, meaning Rammus only has one disengage (Powerball) instead of two (DBC and waddle back to turret). This reduces my safety. Instead of DBC always being the answer, it gives me a decision space to solve: "Is DBC worth the speed debuff? Will I take more damage crawling to safety than I would walking normally?" It gives me a clear moment of weakness when the CD is off, but gives me more control over where that window is with the new early-off option. It also fits well with Rammus' target fantasy: tanks are slow, so if I'm tankier, I should be slower. It may have gone so far as to damage the viability of top Rammus, but that's probably okay. I was abusing a toxic play pattern to begin with, and I'm not sure Rammus was meant to go top. I'm sure the Riven in the vid I linked to was not having fun against me, for instance.