The setting.

I didn't buy the game because I liked the rules...I do, but that is secondary to the really cool setting.

I like HG Wells and his stories and to combine them and make a wargame from the extrapulated tech.  Awesome.  But the history that develops from the War of the Worlds is really well thought out and fun.

I like the rules because they are simple but not simplistic.  There is enough there to make each game challenging.  I don't need a bunch of templates or a hex grid.  I just need a table, some dice and a ruler.  It's great table top fun.

But it's the setting that rules.