So, basically, a nukige is the complete opposite of something like Crescendo. Crescendo was highly story/character driven, the majority of endings didn't involve sex (and many of those were really good), and was just beautifully written. A nukige is basically just "wham bam thank you ma'am, get the sex scene as fast as you can."
Okay, got it. (aside: Crescendo is one of my all time favorite visual novels.)