I agree with the OP to an extent.
I've been toying with the idea to upgrade ever since the iPhone 4 was announced but in the end I put things into perspective - it's a damn amazing phone, there's no denying that, and definitely the best iPhone yet. But a lot of the features I, personally, wouldn't make use of.
I have no issues with the 3 megapixel camera in the 3GS, because they're just casual photos and nothing professional for me. I've only used video recording once, and that was just a casual thing when I was out one day. No need for a HD camera or iMovie or anything. The forward facing camera? Taking photos with it would be alright, but stuff like FaceTime... no point when there's only one other person I know that has an iPhone to begin with.
The gyroscope...well, there's only one or two games I really even use the accelerometer for (Super Monkey Ball basically), and I've even turned the 4.0 rotation lock on. I don't see why I'd need another motion sensor when I barely use the first one to begin with.
People could say the new processor is another plus, but in the end 4.0 on the 3GS is working flawlessly for me - it's snappy, it does everything I need it to do, and it does it well.
The screen is probably something I can't really comment on until I see it in person, but I don't have any real issues with the 3GS screen. Sure you can clearly make out the pixels (just like most devices with a screen), but I can still see everything I need to, it doesn't look bad.
802.11 n support... my internet connection isn't even close to being the speed that a 802.11 n router supports, useless for me.
Improved battery life - the 3GS battery has never been a real big issue for me, and 4.0 seems to have improved it a bit for me.
The second mic - kind of useless to me to be honest. I text way more than I make phone calls (in fact, in the 10 and a half months I've had this phone I've only used it for calls for 5 hours and 53 minutes). The main appeal for me when I got this phone was the tethering and the apps - the iPhone 4 doesn't improve on these (obviously the iPhone 4-only apps excluded 🙄 )
The features I already have far outweigh the features I want - I'm not going to put money down for a new phone and commit to a new 2-year contract just to get a camera flash, forward facing camera (for photos only, in my case) and for a new-look case.
But basically in the end, it comes down to what the user needs/wants. If I wanted the iPhone 4 then I would upgrade whether I was eligible or not (or buy it outright and unlocked from Apple Australia to avoid recommitting to *shudder* Telstra). Then again, this is coming from the same guy that was perfectly happy with my iPod touch and impulsively bought a 3GS when, earlier that same day, I hadn't intended to.