In no particular order:
1) its based on a good little graphic novel
2) as Norm says, it has Maria Bello and Ed Harris in it (for some, the draw my be Viggo Mortensen: I don't get that myself, I prefer the acting qualities)
3) it is actually rather violent but any 'pleasure' the audience might take from the violence soon gets undercut. There is a good review in Sight and Sound which HAS SPOILERS (the link is here), but I'd also urge anyone interested to read the article by Graham Fuller from the same edition of S&S that is unfortunately not available online. The film is not quite what it seems, and its ending rather reminds me of the final song from the musical Buffy episode: "Where do we go from here?" What happens when violence becomes the means to end/escape violence and its history?
Thought provoking stuff.