In addition to a fact I think I'd omitted (my chosen worldgen options often take me to the year 1080 (as I don't interrupt them early) or so, for my embark to occur at), I suppose have a small query based upon some of what's been already mentioned...
When I first started this game (prior to 40d, not sure exactly when though without mapping my earliest posts here with the version history) I seemed to get quite a lot of 'canyon' embarks. ISTR quite a few where there'd be a mountainous 'top' with a river down a deep gorge and a wide (4-5xRiver-width) flood-plane level with the water surface dotted with rocky stacks, obviously the product of river diversion and re-scouring over time, during the worldgen process.
Although these days I seem to be actually hunting for more plane-like embarks, for my long-term projects, whenever I've actually looked for high Z-varying riverside embarks in rocky terrain (which is where I'm sure I've previously found my canyons) I'm getting very little that looks like that. Even looking for * and *+ed 'height variation' indications in the appropriate embark-site tab. I've played at times with the "periodically erode" setting, thinking that maybe it should be unset (to retain vertical rock-faces, rather than make them into merely 'steep slopes'), but maybe there's been an underlying change in the terrain modeller that's precluding these setups.
Or perhaps, it's because I'm letting it go for too long. A 50-100 year history gives canyons, but let it last until 1080 or so and (regardless of erosion settings) it all gets made rather more 'rolling'. in the same locales. If that's the case, I might spawn a few still-huge worlds but with an enforced short history, in order to play about with similar worlds as I remember from my 'youth'. But if anyone knows for sure, please feel free to let me know what I'm doing wrong/not right...