I think The Hunger Games novels are great reading for any children old enough (and intelligent enough) to comprehend them. My 12 year old son is currently reading them. But yes, they don't dumb down, and adults can and do enjoy them too.

I also think the films could have done with being a bit gorier, perhaps aiming more at an R rating rather than PG-13. Here in the UK, the film was censored to get it to a 12A rating in cinemas (equivalent of PG-13), but the uncut 15 version (the same as the US PG-13 version) is also available here on Blu Ray. Even in the fully uncut version, I think it needed to be a bit nastier. However, the problem is far worse with the other two films. They really needed the gore that the books have, and it is all but absent.

There are other aspects of the novels that I miss in the films - the level of detail concerning the hunter/gatherer stuff, for instance - as well some of the darker edges. Peeta kills someone in the games, and that is removed. Katniss later kills a stranger in the Capitol (in Mockingjay), which is a key moment of character development - that's also been removed. There are other minor aspects I miss - the Avox stuff, for example. Also, I do think the final scene of the final film really doesn't work on screen. Too upbeat and cheesy. Whereas on the page it worked, because of Katniss's inner monologue about her trauma, etc.