It's too cold to survive in the winter: Winnipeg, Anchorage, Oslo, even Chicago (solution: clothes, weatherproof shelters, heating during winter)
Nothing grows there, it's too dry: Saudi Arabia, San Joaquin valley, etc. (solution: irrigation or importation of food)
There's no industry: anywhere on Earth before humans lived there (solution: build industry there)
In comparison to the leaps that humans have historically achieved, the leap of 21st century man to colonize Mars is practically easy. Polynesians ranged across the Pacific Ocean, not even knowing what they would find until they got there.
Yes it will be hard. Uniquely difficult in the history of mankind? No. Impossibly hard? Also no.
Worthwhile? Almost certainly.
You don't need to terraform Mars to make it habitable anymore than you need to terraform Oslo or Anchorage. You build your own habitats, you make your own air just as today we make our own food and sometimes make our own potable water (using desalination). The difference is merely one of degree. As the 21st century advances our technology will advance as well. People will come to rely on technology producing air and water and food just as today we rely on technology to provide electricity, shelter, heating, and water, and food.
The key fallacy here is taking our present state of living as somehow "natural" or non-technological. Humans in the developed world live at the apex of a pyramid of innovation and technological invention that in some cases is only a few years old, and built on a web of industry that spans the globe. It's easy to take such things for granted and imagine that we could live simply without being so dependent on modern technology and industry but that's only a pleasing falsehood, we're quite dependent on modern technology and industry. Without it millions would die, and many settlements would not be feasible. Indeed, the current population levels of California are not sustainable without substantial modern technology. The same will be true for people living on Mars, the only difference is that they won't have the illusory fantasy of believing otherwise available to them.