Positive: melting ice. Because ice is light-colored and reflective, a large proportion of the sunlight that hits it is bounced back to space, which limits the amount of warming it causes. But as the world gets hotter, ice melts, revealing the darker colored land or water below. The result is that more of the sun's energy is absorbed, leading to more warming, which in turn leads to more ice melting - and so on.
Negative: Surface temp increases, increased evaporation from the oceans, more low clouds in the atmosphere, reflects more sunlight into space, surface temp decreases