Terraforming is an interesting topic!
A problem with Mars is that it doesn't have a protective magnetic field shielding it from radiation such as the solar winds and this continously strips away the uppermost atmosphere, making the atmosphere comparatively thinner than on earth. Also, you get more harmful radiation for organisms potentially living on the planet which leads to potential problems like harmful mutations and desiccation.
Before you could put organisms on the planet, of course the abiotic conditions would have to be so that they could survive. It would be nice if Mars was warmer, if the ice melted and you got water vapour and thus a denser atmosphere etc., but I guess you would have to find some way of keeping it warm and no magnetosphere still means it would lose gasses to space continously ..
But it's an interesting topic. Just imagine a Mars where all the oceans have melted and are liquid water again. We can still enjoy it in our imaginations ..
edit :
Ugh, plants on Mars .. I'll believe it when they actually find them and not just pictures of something someone thinks looks like a humongous leaf lying on the ground in a picture taken a long way away from the planet surface.
For a real photo of Mars, check out
this picture taken by the Mars Exploration Rover Opportunity and shows the view of Victoria Crater
