So presumable, too generalise, a hole that has no bunkers, no ponds/ditches, no trees, no rough etc would have a very low slope rating whereas a hole with lots of such features would have a higher slope rating?
Pretty much. But… length is still a predominant factor. The scratch golfer hits the ball 250 and then 220 in the rating system, while the bogey golfer hits it 200 and then 170. So any hole that's about 430 yards, even if it's wide open with no trouble, will typically rate a good bit higher than a hole that's 350 with a few bunkers for the bogey golfer. It takes them three shots to reach the green, after all.
That is correct. Slope reflects difficulty for the bogey golfer. The higher the slope the more challenging the design. As I said earlier, a shorter course with hazards placed out of the way, with greens that are not treacherous, with limited water hazards and/or deep penal bunkers, and minimal if any long forced carries,... will have a much lower slope.
To nit-pick, the slope requires two points as all lines do, so a 74.5/133 course might actually play a good bit "tougher" to a bogey golfer than a 70.1/137 course, especially if both are par 72.17.6×133/113+74.5−72 = 23 CH
17.6×137/113+70.1−72 = 19 CH
Slope is "relative." The NCRDB lists the bogey rating these days, so I'd look more at that if you want an actual overall "difficulty" level for a bogey golfer.
Aronimink from the blue tees: 72.2/130, with a bogey rating of 96.3.
Pine Valley from the front tees: 71.9/150, with a bogey rating of 99.7.
Seminole from the white tees: 69.7/129, with a bogey rating of 93.7.
And finally… Bandon Trails from the greens: 71.1/129 with a bogey rating of 95.0.
The last two for example have the same slope, but BT plays tougher for the scratch and bogey golfer.
That said, they tend to track pretty closely, of course. You're not going to find many 74.3/112 courses, nor are you going to find many 67.3/148 courses… because length is the primary (by far) determinant of the course ratings for both scratch and bogey golfers. (I'd be a bit surprised if courses with those ratings exist at all, and if they do, what they're like, and what drugs the rating committee may have been on at the time
; that said, the latter is more likely, I think, because again the slope is relative, not absolute).