I didn't say my opinion was the be-all, end-all. It's just my opinion.
Last year, the first episode in December (Loco Motives, Episode 10) got over 23 million viewers. So it's hard to blame it on the time of year.
The writer's strike? Maybe, but wouldn't people at least check to see if it's a new episode? It seems odd that people would go, "Oh, writer's strike - I'm going to completely not turn on my TV for a few months."
From their last non-holiday episode (episode eight), GA dropped 10%. CSI dropped 13%.
From their premieres, Grey's is down 15.7%. CSI is down 26%. So for GA to be at 17 is not nearly as significant as CSI being at 18.