Mars has been on the books as the next step after the moon for a long time. Some people think that our next step towards space needs to center around going to Mars.
I'm going to write about why this may not be the great idea you think it is.
First, technological troubles. NASA likes to ignore these. You need new Mars-specific spacesuits because the insulation required to keep the astronaut at a comfortable temperature is tricky. The lightweight design they use for orbit and the moon won't work. Mars has enough gravity that you can't just layer many normal forms of insulation. And the size and shape of mars dust is going to get into the works.
Landing large payloads is hard as well. Not enough atmosphere to use parachutes to do all the work, but too much atmosphere to get away without heat shields. We also don't know what a sufficiently long mission will do to humans. The zero gravity portion is fairly understood. The radiation environment is much harder to plan for. Furthermore, we have next to no real experience keeping a space vehicle going long-term without any resupply from earth. Because whole replacement assemblies are easier to ship up from Earth, that's what the ISS uses, so we really don't have much experience on what would happen if we flew to Mars.
None of these are things that ought to make it impossible, mind you. It's just that it's not as simple as dusting off our lunar exploration designs and finding a way to manufacture them. If there are other missions to be had, there might be more useful goals for the short term.
Second, no useful later goals. Most of the technological troubles are specific to Mars... but at the same time, it's easy enough that we could make a mars-specific suit, a mars-specific lander, and a spacecraft made from existing known good systems. If we wanted to land on asteroids, mine the Moon, place balloons above Venus, build a space colony, etc. very little of what we develop for Mars will work there.
Third, potential lack of concrete goals. One point made by many people in the space exploration communities is that obsessing about visiting planets is really getting in the way. A space colony is also possible. It will take less energy to get there from Earth. You can tailor the environment to match our needs exactly there. The problem is that if we make a big push to get to Mars, it might end up the same way that the Moon is. Footprints, flags, and only a few missions.
Finally, life on mars. This could get incredibly sticky. See, we've rapidly realized how quirky life can be. It's starting to become plausible that an extremophile form of life from Earth might survive on Mars. It's also plausible that there might possibly be life on Mars. We don't know for sure, of course. Apollo astronauts brought back parts from Surveyor 3 that had bacterium on them, but it's uncertain if that means the bacterium were placed there when the astronauts picked it up or if they survived being on the moon for 3 years.
See, it's not presently provable that Mars does or does not contain life. It had liquid water and a more gentle biosphere in the past. We just don't know enough, both about the range of extremophile life and also about the exact nature of Mars.
But, let's say that we find life on Mars. See, one of the things that supports the theory of evolution is that, on a molecular level, every single form of life works exactly the same. The same DNA molecule, the same language of protein coding, the same chirality, etc. So, given a sample of life, we can examine it and see how similar that form of life is to Earth forms.
If it differs in some major way, we can assume that it formed separately.
On the other hand, if we find that it works exactly like life on Earth and maybe even contains shared genetic material, we've got a problem. See, there are credible, non-disproven panspermia theories that suggest that life could travel from Mars to Earth or Earth to Mars via meteors or that a form of life from elsewhere rained down on our planet. If we were to accidentally introduce a terrestrial extremophile form of life, that will confuse things.
Furthermore, if there is life on Mars, we've got more problems. First, we don't know how a completely different life form will interface with us. It's almost beyond the range of imagination. We've got a toughened coat of proteins on our skin that repulse most micro-organisms. Maybe it won't repulse mars micro-organisms.
Furthermore, we might accidentally introduce extremophile competition. Humanity sorta feels bad about the dodo. How about wiping all life off of a planet?
The problem is that the people who say that we should shut down the manned space program and send robots instead are only half right. We are getting a lot of interesting research at the moment with our Mars probes. But just because modern Mars probes can do much more than they might have in the 60s doesn't mean that there's no reason to send humans to space.
I often think that the biggest and most persuasive Mars mission for me these days isn't actually landing men on Mars. It's landing on Phobos or Deimos and using that as a base of operations to command sample-return rovers. If they just need to get samples up to Phobos, that's much less delta-V than getting them back to Earth and being able to have a quick turnaround on the samples is going to enable more research to be done in less time.