By now you have probably heard media reports of a 'Goldilocks' planet orbiting Gliese 581, the
seventh sixth planet found in the retinue of this dim red star.
Mass media hype aside, this discovery is both important and unsurprising. Important because this is the first known planet apart from Earth itself that orbits entirely inside its parent star's habitable zone, and so could potentially harbor life without broiling or freezing it. Unsurprising because nearly 500 extrasolar planets have now been discovered, and sooner or later one was going to turn up in the right orbit.
All we know about Gliese 581g are its orbit, at about 0.15 AU from its parent star, and its approximate mass, about 3-4 times Earth's. We do not even know for sure that it is 'a planet' rather than, say, two planetary-mass bodies orbiting each other. We know nothing about its composition, such as whether there is water vapor in its atmosphere, let alone liquid water on its surface. We know only that liquid water could exist at that distance from the star, unless the planet has an intense greenhouse atmosphere or some other complication.
But let the speculation begin, as naturally it already has. If it is a single body it should be tide-locked to Gliese 581. This used to be a deal breaker, but current thinking is that atmospheric heat transfer is ample to keep the air from freezing out on the nightside.
If the planet has extensive uplands and limited water, the water might (my own speculation) form vast ice sheets on the nightside instead of pooling as oceans. On the other hand, the general feeling seems to be that big planets will have more water, while the heavier gravity should make a rocky surface flatter - how much so is above my pay grade to estimate. But this may well be a waterworld, or even a 'water giant' with a hydrosphere thousands of kilometers deep instead of Earth's thin muddy film of liquid.
As Arthur Clarke said of Jupiter, when it was thought to possibly have a deep hydrosphere, think of the fishing.
I would not rush out to put a colony on Gliese 581g in my setting. It is probably not a world for us. (If any worlds are 'for us' beyond the one we evolved on and any we may one day terraform.) But we are free to imagine a golden-red glint of sun across a very distant sea.