Had the interesting experience today of getting on the trainer after something like 6 straight weeks of only outdoor riding, and a half dozen races. So my fitness is way higher than the last time I was on the trainer.
After warming up for 45 minutes I set out to do 2x20 at ~90% FTP. I've done quite a bit of FTP type work in the last 6 weeks, so I'm pretty tuned in to what that feels like.
And on the trainer (Revolution) ... it feels so very different than being outside. I realized that part of the "issue" is the bike being fixed to the trainer. I wanted the feeling of being more attached to the bike, with the bike moving with me, as opposed to me moving around on the immobile bike.
I also noticed feeling the effort being so concentrated in my quads, and my legs started feeling a fatigue that I would never feel at the same power level outside. Nothing new there, but the sensation was just so obvious having ridden outside exclusively.
I then noticed that if I moved forward a bit more on the saddle and used one gear bigger, with a slower cadence, I got more of a road feel ... and could hit a higher power level. So that is perhaps something to remember and try again.
My own conclusion still remains that riding the trainer makes me better at riding the trainer. Not to say that trainer time is wasted effort, but I think it's telling that after 6 weeks of training and racing, my 2x20 watts on the trainer was pretty much the same as when I did it 6 or 7 weeks ago (while what I've been doing outside has been going up).