What was most fascinating for me about this classic was how fascinated I was by it.
This is straight-up hard sci-fi, no bones about it. Yes, it's also military sci-fi, and that's important in how it relates to other military sci-fi that came before and after, but at its core it is written by someone steeped in science. Because of how it's told, we feel a huge amount of empathy with the grunts, even when those grunts move up the chain of command. That was fascinating to me, because it's hard to write sci-fi (or anything) from the perspective of the "nobody" when that nobody keeps gaining more power.
There are some strange references to homosexuality for about half the book, and then it gets serious about describing the difference between homosexuality and heterosexuality in the future. At first, I was a little horrified at how dated the book was feeling, and then I was bemused, and then I was fascinated. While it absolutely, definitely, no question is written by a heterosexual, it tries, in unexpected ways, to be open to differences in sexual orientation.
And lastly, it was fascinating because I was happy about the ending. Why should I be happy about this ending? It's a surprising way to finish a novel with a bummer of a denouement. It makes it seem as if we should be pleased about the handful of folks who've survived the entire plot. I think Haldeman knew that the readers fully understood the horrific nature of the plot, and that it wasn't worth drilling that home any more than needed. In the end, at least someone gets what they've deserved, and that's worth celebrating.