IME the big thing that separates trainer from road (aside from boredom) is the lack of wind. No true hills is a biggie, too. All this past Winter I did 50 or more miles/week on the trainer, thinking that would keep me sorta bike fit...but it didn't. At least not as much as I had hoped. It didn't help that we had the neverending Winter and I wasn't really able to get out on my bike much until April. I had about 6 months of indoors-only and it was miserable.

On the bright side, my fitness came back pretty fast once I was able to get outdoors more. And now I have a cyclocross bike so that I can get outdoors for rides with studded tires when there's snow and ice.