The chemical content of the rocks the rover is able to analyse comes close to matching the chemical content of the rocks we are finding on Earth that we think came from Mars. The biggest find so far of the Mars orbiter is that Mars has a faint magnetic field. This is a significant find for a strong magnetic field can turn away deadly radiation from the habitable protion of the planet. Without a magnetic field protecting life on earth, radiation would bombard us directly and it is unlikely that most life could survive. We have also noticed that Mars' weather is more complex than we thought.
Temperatures vary more than they do on Earth.
The weather is more global than it is on Earth. On Mars, if the weather is windy and dusty on one part of the planet, it is likely to be that way on the rest of the planet. That pattern is not true on Earth. We cannot tell what Mars' weather might have been like when it had moderate temperatures, lots of water, and a dense atmosphere. When we got our first close look at Mars in 1965, we decided it was more like the moon than like earth.