While I also prefer 8043 to 42009 I don't think it's such a stretch to cite the mobile crane as a "landmark" set. I would say that some of it could be attributed to not being able to purchase 8043 or even see it in person to compare it with 42009. I can't speak for those in this thread but if they were only seriously thinking of getting back into Lego in the last couple of years then a £200+ out-of-print set was never an option. If you had made this thread before 42009 I suspect we'd see a similar trend with 8110 (replacing 42009) and 8043.
You've also got to factor in the sheer part count of 42009 - it [currently] has the most parts of any Technic set. That's a big draw for tempting people out of a Dark Age and back into a hobby which is known for being expensive at the best of times. YMMV on how much value for money the 42009 parts actually represent, but the "endless pins" debate is probably best had in another thread. Again, the numbers argument extends to the boom extension and I think the number of axles/wheels? (Correct me on this if necessary). As with most things nowadays the headline figures of the crane grab people's attention, perhaps moreso than anything individual aspect of 8043 at first glance. Add that to the assumption that bigger => more complex => better and it's not hard to see why 42009 is so well received.
(I should point out that I do like 42009 a lot, despite just denouncing it in the above paragraphs!)
As for the set that brought me out of my Dark Age, it was 8110. I had the choice between that and 9398 but the crawler was never going to win; I had the chance to finally get a pneumatics set that my parents never saw fit to get me as a child...
Also first post, hello, AFOL, Technic fan, etc.