Well, Mars is so much farther away and much more massive so you need a lot of fuel if you want to come back. This is much more difficult than the extra fuel needed for the moon landing due to lack of atmosphere. Speaking of which, Mars having an atmosphere means you need complex heat shields for the landing.
Furthermore it's so far that unlike the case of the moon you can't make real time adjustments from earth, there's a delay of several minutes. Then again you have dust storms...
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Atmospheres make landing easier and require less fuel! A big problem with landing is losing your speed which is going to be extremely high to begin with. On the Moon you can only do this by basically turning around in the opposite direction of your velocity and thrusting an equal but opposite amount. It's not only quite complex but also substantially complicates landing.
This is made even true on the Moon because its low gravity means that even a hair of velocity is going to make you 'bounce' after landing. This is why things like probes and rovers landing (or at least ending up) on their side or even upside down on the Moon is a fairly frequent affair. On Mars (and other places with an atmosphere) you can use atmospheric braking which is essentially just slowing down by bumping into the atmosphere in a controlled fashion. You can even get things like parachutes involved in the process.
The dust storms in Mars are also 'fake' at least as presented in movies/books like "The Martian." Mars has an extremely low atmospheric pressure (relative to Earth) so fiercest dust storm imaginable would feel like nothing more than a slight breeze. The only issue they pose is visibility, and dust accumulating on solar panels. Andy Weir, by the way, was well aware of this when writing "The Martian" which is otherwise a phenomenally well researched hard sci-fi book. I think it's highly telling that he had to intentionally fudge reality to create a crisis on Mars!
I think the big piece that is being overlooked here is the distance. The distance itself poses significant challenges. The obvious things like resupply and communication are much harder. But also the journey to mars is much harder on the human body.
Rescue and abort options are also much harder. The moon is close enough to easily resupply or rescue people on the surface, mars is much harder.
Completely agreed. Distance will impose substantial challenges, but the good thing is that that's really the "only" big challenge there is. I think many people have this mental model where the Moon is easy and Mars is hard, perhaps because we've already set foot on the Moon and so clearly it can't be that bad.
But if somehow both of these bodies were orbiting around Earth, Mars would be just orders of magnitude more straight forward than Mars, and I think it's relatively likely we'd already have permanent outposts, if not colonies, there. So the mental model of it being viewed as a stepping stone is somewhat misleading. The Moon is hard!
And also I don't think the distance will be that bad. We've already had 374 day ISS stays which is far longer than any possible transit to Mars (though nowhere near as long as a late-stage mission abort would entail) and the overall effects of such a stay were not markedly different than significantly shorter stays on the ISS. So it seems very unlikely that even a late stage emergency abort would be fatal.