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Originally Posted by Moose
The challenge on Mars is that you really don't have the basic building blocks in place in sufficient amounts to reach "Earthlike". No liquids (let alone water) that we know of.
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Well, we have seen some evidence of liquids on Mars. More importantly, there's a good chance there is still quite a lot of frozen water under the surface. Of course, we need a lot more information on this. Assuming there is a lot of ice, it would take time (a long time) to unfreeze a significant amount of the permafrost. But, even given the uncertainties, Mars does far better than Venus in the water department.
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The atmosphere (such as it is) is carbon-based, but so thin you'd have to haul in what you needed (and eventually lose it all to outgassing anyway). Much less energy provided by the sun.
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Atmospheric loss takes a very long time on human timescales, so that wouldn't be a significant issue for us. Producing atmosphere would be a biggier issue, and it is unclear if there are sufficient materials to produce a good atmosphere on Mars (I suspect importing would be a requirement). Temperature is a major problem and would be a key reason there would probably need to be permanent artificial intervention. Of course, Venus has the opposite problem: Too much atmosphere (and it isn't easy to deal with) and too much sunlight. Then there is the slow rotation rate.
One other point: Since Mars is smaller, the scale of operations (solar reflectors, mass moved, and so forth) would be significantly smaller than for Venus. Still REALLY, REALLY BIG by present standards, but smaller.