There are a lot of problems with a manned Mars mission:
1.) Gravity. We have a fairly good idea of how to reduce bone loss and muscle weakening (vigorous, regular exercise), but even so a round trip to Mars will be (IIRC) the longest time any humans have spent in zero-G. Furthermore, we have no data on how how the human body will respond to 1/3 Martian gravity over a standard 500-day stay on the surface. Then there is the issue of radiation from solar storms, cosmic rays, and the like.
Mars' higher gravity also makes any manned mission much more difficult than it would be for the Moon.
2.) There's a lot we don't know about Mars itself. The nature of the (probable) peroxides on its surface are unknown. How toxic are they? How corrosive? Will the dust cling to suits and gears, gumming up machinery? *Does* Mars have some pathogens that could be deadly to humans? Only a sample-return could answer that last one. Is the atmosphere sufficient protection from solar storms?
3.) Living on the surface. How will we generate power? Nuclear seems to be the only realistic option, though that will greatly increase the mass we need to send to the surface.
4.) Psychology, as someone already said. A Mars mission would probably last two years at least, and possibly as long as three, depending on the profile. We know it can be stressful for astronauts orbiting the Earth on long-term missions, an environment where they at least have instant communication and a ready escape valve if anything goes wrong. But two to three years of Earth being nothing but the proverbial pale blue dot, with a long communication lag, and knowing you're stuck with 4-6 people that entire time?
I think it's safe to say that even with modern communication, and with their own companionship, the people who first go to Mars will be the loneliest who ever lived. I have too much faith in the human spirit (and psychological screening
) to think that people will go bananas on a Mars mission, but I think the possibility of severe stress reducing the crew's performance is a real one.
5.) General risk. The success figure for unmanned Mars missions is pretty dicey: 50%. I think a manned mission will have better odds, but it will be a tough sell. When an unmanned mission fails, it's something that gets a few laughs on Jay Leno and in political cartoons. When a manned mission fails, it's a National Day of Mourning, there are hearings, finger-pointing, endless bureaucratizing, paralysis by analysis. A failed manned Mars mission would be the sinking of the
Titanic next to the
Columbia's skiff. If the failure happened on Mars or in transit, we might never know what caused it, making it all the worse.
In short, this is something that could set back manned exploration for decades, if not permanently. They say with great risks come great rewards, but when votes, national prestige, and tax dollars are on the line...people get a little skittish.
For my part, I think R&D in novel propulsion techniques--nuclear thermal especially--would go a long way towards easing some of these problems, by increasing lift capacity and reducing transit time. But given a history of "if it ain't broke, don't fix it"-ism in NASA, I imagine we'll still be taking the slow boat there, if ever.