Some people have said that Muslims have a right to be upset over some of the things in Charlie Hebdo. I think this is incorrect for two distinct reasons.
The first of these reasons is the fact that satire is a useful tool. Being in poor taste does not make something offensive of itself. Let's look at art done about other similar topics.
Let's take for example a comic such as this, this, this, another, and so on. Any of these could easily be as offensive, to a particular person, as any of the cartoons about Muhammad, for example. This does not mean that they are offensive of themselves. The cartoonists are not bringing hate upon themselves for pointing out things that are actually happening. Just because some people who sympathize with particular viewpoints are offended, doesn't mean the content is offensive. Each of these can be seen as offensive, but that doesn't justify hate crimes.
The Hebdo incident involving an anti-semitic journalist is different, because that fellow did, in fact, engage in hate speech, by saying a class of individual should die (note that terrorists tend to think this way as well, also Muhammed teaches to behead infidels [and islamic states have death penalties for apostates and atheists], and Christians have burned 'sinners' at the stake, and sent unbelievers to hell). Pointing these things out, however, is not offensive, especially in the context of satire, because that is what satire is for. And again, if you're offended that I pointed out that some followers of Islam do indeed want to kill anyone who doesn't believe, or that some Christians still want to send heretics to hell (or punch in the face, as the pope so eloquently put it, anyone who 'offends' the faith [by discussing these things?]), then it's probably not the fault of my speech that you are offended.
But let's move on to that second reason. This one is a bit more subtle, so let's work it backward.
Let's take a picture like this one:
But putting aside that, one picture is literally an affront to the other. A depiction of Muhammad is, essentially, also creating a graven image of a false idol under scripture of the bible. Similarly, painting Jesus in stained glass is clearly an offensive front to any follower of Islam, as that would clearly make one an infidel, apostate, or blasphemer - implying some other god exists.
This leads me more directly to the point: anything that goes against what you believe can be offensive. PETA, for example, have done some good work in uncovering a few instances of negligence and abuse, but also routinely kill animals at their own no-kill shelters.
We are Charlie Hebdo. We all have a right to express our disdain for things. We all have a right to poke fun now and again. We all have a right to make people uncomfortable. We don't have a right to be violent, however. We don't have a right to simply kill people we disagree with.
Then again, there's a third, even more subtle point here: we are all human. Why must we subdivide? Would this be less offensive if it were Kim-Jong-Un (wishing everyone a happy new year, including his 200k or so enslaved citizens)? Would it suddenly be more acceptable? What if it were the pope (wishing well all those people he'd punch in the face for calling Jesus an etherial skydaddy), or maybe Charlie Manson (wishing his victims well from his prison cell)? Are those really more acceptable satires? Are they any less offensive?