A lot of interesting themes here IO. To go back to your original question of why fewer "serious" threads.
I think there may be a number of themes at work. One has been identified as the one-off nature of internet forums being a "new" thing that many people got involved with, but over time, as with many new things, interest wanes and people drift away.
Another theme may coincide with the emergence of Internet pilot fora in the last decade. The 2000s were a period of change in GA, after a fairly stagnant period in the 90s. There were simply a lot of "serious" new topics to discuss
- new aircraft like the Cirrus (and new concepts like BRS) and diesel Diamonds
- new avionics, initially the GNSx30s and then the G1000
- new methods of navigation, everything from handhelds to GPS Approaches to PRNAV
- new regulatory issues like EASA
In the last couple of years, the pace of change has slowed a bit. Many of these topics are now mature. EASA has sort of settled, although I am sure there'll be another flurry of IMCr posting at some point. The recession has bitten into new aircraft development and a lot of projects have stalled/collapsed.
...perhaps there inherently fewer themes for "serious" threads at present?
The other inherent issue is that many "serious" topics are by definition specialised. GA often works well as a nice community, but when it comes to fora, it also fragments and tribalises very quickly. Rod1 thinks mainstream GA is over-represented compared to light sport flying. I think there is too much kneeboard/torch/hobby stuff and mainstream GA topics are a bit under-represented. To be honest, I am not terribly interested in the homebuilt perspective on every topic, and I am sure Rod1 isn't terriby interested in the certified perspective. It's the problem of an "all private GA forum". You gain a good volume of posters and readers, but most of the time you fail to please most of the people.
Also, an anonymous general forum is not ideal for a serious discussion. For example, if I were a Cirrus owner and I had a "serious" question, would I post it here? No. I'd get too much noise - the jibes that I must be a rich b*****d, the "Cirrus BRS and glass cockpit induce complacency" knee-jerk, the "certified aircraft are dinosaurs, fly a homebuilt"....etc etc etc. I see a lot of Cirrus aircraft around the UK and Europe, which is why I disagree with the "GA is declining" argument (the Cirruses are always flying too, so the airframe #s probably underrepresent the usage). But I imagine Cirrus owners join a Cirrus Pilots forum.
The problem is even worse on the main airline forum. I dip into it on occassion to read the pro pilot perspective on interesting issues or on an accident, but the pros are drowned out by hundreds of other posts and you give up wading through 50 page threads.
brgds
421C