I went to a special screening with my two kids: a 16 year old boy and 10 year old girl. I was shocked at how violent this movie was, and I am one who usually scoffs at "cartoon violence". Yes, it's a cartoon, but this movie in particular has very realistic, very angry violence that has a different feel than Tom & Jerry or any Disney movie (really, they all have violent scenes). My 10 year old daughter was made uncomfortable by it, and just the night before we had watched Charlie's Angels. She is not sheltered from violence, by any means. Mazo997 is right. There is every kind of aggression and weapon used in this movie. Read Mazo's review for a more detailed description, but in ADDITION to all of that, there was even a joke about someone being hanged (accompanied by a visual).
It also bothered me that a couple of characters cross a very busy street by blindly running through it. One does get hit and bounced comically but painfully around many cars for a verrrry long time. This is a personal pet peeve of mine, and why I added "Not good role models." Some people might also be disturbed by the old west style of living (lots of drinking and yelling, everyone and their grandmother has a rifle, etc.) but I let that go since it is, basically, a Western.
I was surprised by the language used: "you son of a __" where the last word is essentially bleeped out, and lots of "h-ll" and "d*mn", all used appropriately, and, again, angrily. Also the language is very adult just in the way the characters speak - think True Grit. My daughter couldn't really keep up with the accents and the manner of speaking once Rango gets to Dirt.
Given all of that, I still have mixed feelings about the movie. My daughter did not like it at all. My son really liked it. I felt that it was trying too hard to be a kid's movie for adults. There have been some examples of balancing the two (The Muppets, Rocky & Bullwinkle, all Pixar movies), but this movie seemed to forget that young kids would be in the audience, so even the scenes that made only the adults laugh (like the Fear and Loathing reference) made me cringe and just hope I wasn't going to have to explain it later to my kids.