O6 wasn't very strong or deep? The 1960s NHL was the best professional hockey league in history.
True that the league was imbalanced, but still.
No, the Original 6 was certainly never strong nor deep in talent.
It had more talent than all previous eras, no doubt. But the overall talent pales in comparison to what came after.
The talent really started to pick up around the mid 1950s, just following Howe, with Beliveau, Moore, Bathgate, Delvecchio, Geoffrion, etc. And it was relatively stable for the rest of the O6.
The 1960s were pretty good overall - it was a stable league, some good defense, integrated minor league systems...but there weren't many talented players. The Habs were strong, the Leafs (who I especially like) were relatively deep for the era. But the other teams were all lacking in depth. There wasn't much offensive talent in the league.
The talent level really started to pick up in the late '60s and early '70s. Trouble was, it was distributed on a greater number of NHL teams, plus in the Soviet Union, Czechoslovakia, and Sweden. Hockey was exploding in Europe, and the Canadian talent pool also improved significantly at the same time.