I think the 18th hole should be a birdie opportunity. I don't care for the holes that you cannot birdie, but is a hard par. I was not a fan of Merion's 18th this year.
Why? If it is a hard 4, tell yourself it is a par 5 and it is then a birdie opportunity.
I care more about the potential for a swing in strokes, without having penalty strokes be likely, or resorting to 'chop it out sideways' rough or trees. By "likely" I mean something like those too numerous finishing holes the tour plays with water hard in front of the green and a Sunday pin position that is inevitably front just behind that water (or back left for all those holes with water running all down the left side) Something like Pebble's 18th, I'm fine with, to end up in the ocean you have to either take a very aggressive line and fail to pull it off or hit a very bad shot - basically, if you end up there, YDI.
What you don't want to see is a finishing hole where over 90% of the golfers of a given skill level are shooting one of two numbers. Whether those two numbers are birdie and par, or par and bogey, the hole is equally lacking in excitement as a finishing hole. Birdies don't make a hole a good finishing hole, a range in scoring possibilities does.