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There isn't anything much wrong with Mars that a decent atmosphere wouldn't fix right up. Unfortunately the place hasn't got one. Looking at Mars's situation from a human point of view—and how else have humans ever looked at anything?—the mean little atmosphere Mars does have has a number of serious things wrong with it. The most important of these is that there isn't enough of it. The air pressure at the surface of the planet is a pitiful nine or ten millibars. That's so tiny that people on Earth would probably call it a vacuum, but it isn't—quite.
That's bad news for would-be ecopoiesis engineers, as the people who practice the fledgling science of transforming other planets into acceptable imitations of Earth have come to call themselves. The fact that Mars is desperately short of atmospheric gases makes their job tough, but there's good news for would-be Martian immigrants, too. The good part is that there happens to be a place in the solar system where all those volatiles that Mars so conspicuously lacks are floating around in vast quantities, going to waste.
That place isn't very handy, but that's all right. Distance doesn't matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go. The source of these potential Martian gases is way out in the fringes of the Sun's family of satellites, far beyond even Pluto; it is where comets sail forever in slow, cold orbits—at least until one or two happen to twitch themselves out of orbit and begin to slide down toward the Sun. The name of the place is the Oort cloud.