As far as playability is concerned hazards can be the distinguishing feature of a golf course but they have to be "hazardous" to be distinguishing, at least as far a playability is concerned!
As for bunkering, it has certainly been one of the architect's most important architectural expressions forever, but certainly not the only one. Mark, things like Pinehurst's chipping areas and greens are not hazards. Neither is the gorse and other danger areas of many golf courses like most of the area off the fairways at Pine Valley etc.
But bunkering is the primary architectural "hazard" over the evolution of architecture. There are certainly many other kinds but bunkering is by far the most common and prevalent.
Probably the most diluting effect to the real function and effectiveness of the bunker hazard is the whole idea of consistency. I'm not sure when it came into being in golf and its architecture but it certainly is here in spades and today there are few courses that really have bunkering that maintains the old time effectiveness of bunkering.
I look at bunkering in three ways--the sand, the structure and the surrounds. If a golf course has all three working well to present the golfer with three types of hazardous playability, then the effectiveness of them radiates out into the player's vision, his thought processes, club selection (distance), aim, just about the entire gamut of strategic considerations.
But alas, many golfers don't appear to want this type of situation or this type of strategy. There certainly appears to be many memberships (or at least the majority of them) that don't. MikeC's remark about Lehigh's bunkers is another good example. Apparently Forse's bunkers (at least the surrounds) were too much for the membership and they had to be changed.
That's merely a choice of preference, but you have to admit they play less like real hazards than they did when Forse debuted his. We can talk about losing balls in the grassy surrounds, slowing up play because of it and scores getting run-up too, but that's what is supposed to happen to make the golfer think more and play harder to avoid getting in them and that fact is the central theme of strategy and basically a lot of the guts of a golf course.
So I think you're correct to a large degree, "It's got to be the "hazards"."