This is my first winter on a trainer, and I'm glad I'm focusing on low-intensity base, because anything harder is MUCH harder. I don't have a power meter to know how much of it is mental and how much is actual, but my low HR numbers and high RPE sure make the trainer seem evil. After combing through Friel's blog post and a few other sources, I compiled this short list of differences between the road and the trainer which make sense to me.

Indoor vs. Outdoor Differences
Mini recovery periods on road due to coasting, cornering, and momentum
Faster deceleration on trainer without mini breaks
Side-to-side rocking of (light) bike on road, rocking of (heavy) body on the trainer
Build up of body heat indoors
Reduced circulation of fresh air/O2 indoors, even with fans
Bike position changes (and altered muscle recruitment) more common outdoors due to varying scenarios