great film but very slow-paced.

Heck, compare it to other police procedurals from that era: The Detective, Madigan, The French Connection. Even Dirty Harry is pretty light on action. These movies are about the nuts-and-bolts of police investigation, and frequently its toll on the police themselves. Fast-paced wouldn't match the stories very well.

I'm afraid that you underestimate the number of subjects in which I take an interest!

By today's standards, yes. By 1968 standards, no.

"Take a film such as "The Godfather" - all those scenes with people just sitting around talking. Long stretches with no action at all."

The amount of "action" has nothing to do with the pace. 12 Angry Men (1957) has no action at all; it's 100% talking, all in just one room no less, yet it holds your interest from start to finish. Bullitt on the other hand, has nothing even remotely interesting going on for the vast majority of the movie. The only good part of the movie is the famous car chase scene (which happens to be action, but like I said, action isn't the only form of interesting content that you can put in a movie). All of the characters are dull and practically emotionless, and the case they are working on isn't interesting either. It's the most boring movie I've seen since Serpico (1973), though Serpico is even worse.

Pretentious adults had no problem forcing themselves to stay awake.

It's called DIALOGUE -

Why does it have to be one extreme or another? People can think a movie is slow without wanting 5,000 cuts. There are scenes where you see McQueen driving, parallel park, get out of his car and lock his door, then walk across the street. Does it make someone less intelligent if they think we didn't need to see all of that?