A comment earlier on in this thread mentioning Pathfinder having more powerful PCs got me thinking.
There are, really, three different gaming styles, all of which are Pathfinder is some way or another.
The first is the PFS environment. In that, a lot of games are at lower levels (with characters sixth level or less). This pretty much corresponds to the lower breakpoint mentioned in another recent thread, where players generally do not yet have access to 4th-level spells. In that bracket the difference between Pathfinder and earlier editions is less marked. PFS scenarios do extend to higher levels, but going beyond 11th level is only possible under exceptional conditions. This, again, is still quite close to the character strengths typical of many old-style settings.
Beyond that are the Adventure Paths, which (if you run them all the way through) might take characters to 13th or 15th level. With the difference in powers (rather more noticeable at these levels) this is getting slightly beyond a lot of the old-style stuff. Level advancement also occurs rather faster than in old school - back in the original days Gary Gygax reckoned it might take three years to get up to those levels, while it seems to be possible to run through an AP at about twice that rate (assuming weekly play).
Even at 15th level, though, you're below another major breakpoint, where PCs get access to 9th-level spells. And you're still a long way below the 20th level, with the "capstone" abilities that provides. To get to those levels (which is what the third style of Pathfinder play strives for) you pretty much have to be running a home game; although some adventure paths offer suggestions on how to proceed further, it's assumed that the GM will be generating a whole lot of the material.
This third style of play is the most different from "old school".
In old school, a multi-season goal might be to clear out a castle, a temple, or a small region (Blackmoor, Greyhawk, Dark Tower, Temple of Elemental Evil, The Barrier Peaks, The Underdark, Dragon Mountain, ...). The BBEG at the end of the final scenario would be a significant threat, but could be handled by a group of heroes (not superheroes).
By comparison, a 20th-level Pathfinder character is far more powerful; most of them would breeze past Elminster (or Drizzt, or ...) without batting an eyelid.
Sorry, but "old school" also had "superheroes" fighting against wide-scale threats. The Slavelords in A1-A4 represented a significant threat to more than just the Wild Coast, although they were in second of the power levels you mention. Similarly, Kalitharius in the Desert of Desolation series (I3-I5) represents a significant threat to the lands surrounding the desert area where the PCs encounter him, even though they are still in the 10th-12th level range when completing the series. Then there is Q1 Queen of the Demonweb Pits, where you fight Lolth (after working through G1-G3 and D1-D3) to prevent her from conquering the PC's homeworld (which she is shown to have done to other worlds); you don't get much more "old school" than the G-D-Q series. If that's not enough, there's always the Bloodstone series H1-H4, going from 15th-20th level if you play out the entire set, featuring (among other things) an assault on Orcus's home and a fight against Tiamat with her consorts; in fact, H4 is specifically designed to handle characters up to 100th level.
Sorry, but "old school" also had "superheroes" fighting against wide-scale threats. The Slavelords in A1-A4 represented a significant threat to more than just the Wild Coast, although they were in second of the power levels you mention. Similarly, Kalitharius in the Desert of Desolation series (I3-I5) represents a significant threat to the lands surrounding the desert area where the PCs encounter him, even though they are still in the 10th-12th level range when completing the series. Then there is Q1 Queen of the Demonweb Pits, where you fight Lolth (after working through G1-G3 and D1-D3) to prevent her from conquering the PC's homeworld (which she is shown to have done to other worlds); you don't get much more "old school" than the G-D-Q series. If that's not enough, there's always the Bloodstone series H1-H4, going from 15th-20th level if you play out the entire set, featuring (among other things) an assault on Orcus's home and a fight against Tiamat with her consorts; in fact, H4 is specifically designed to handle characters up to 100th level.
Not to mention the old 2e boxed set modules, which were like 3 to 4 book adventure paths + map folios + players guides in one handy purchase. Several of those had the PCs taking on threats that spanned worlds and planes.