Yeah, the difference is that back in the day, you didn't know you were in the middle of a story arc. You didn't know what a story arc was, and the comics weren't making it painfully obvious. You just read comics. Each one was good by itself AND as part of a larger tapestry.
For example, I love the "cobra island saga" as I call it (circa 37-50), but back then, I didn't know I was reading a long-form story. In one issue the Joes chase cobra across the gulf in the freighter. In the next, cobra tricks the Joes into bombing the fault line. The next few, the island becomes their new home, Ripcord goes looking for Candy, he fights with Zartan, Storm Shadow dies, lots of battles happen, etc. In reality, it probably did last for a whole year, but so much stuff happened, it never got boring. And I didn't see any individual issue as "part six of twelve". It was just another issue as the adventure continued month after month.
I suspect part of it is that you never knew when story beats were starting or ending. You didn't know that the comic in your hand was the issue that would conclude thread x, start thread y, and continue thread z in the background. Now that everything writes for the six-issue trade, you know that parts 1-5 are going to be setup for the big finish, and you just pray that the big finish actually happens. It sucks the fun out of it quite often. I would just like to read an ongoing story where each issue is entertaining by itself and tells what happens next in the lives of these characters. Life does not happen in neat six-part chunks.