With today's news from NASA about arsenic-substituted microbes from Yellowstone, I've decided to vent about an issue that has bothered me for many years. Nearly everyone that talks about alien life commits horrible acts of anthro-bias. The most annoying canard is the assumption that life will require water. Astronomers focus their hunt for planets in the 'goldilocks zone', not because it might harbor nice planets for humans to live on, but because it is somehow more likely to support any kind of life.
I don't see any strong reason to assume that alien life will resemble life on Earth. Perhaps Star Trek polluted everyone's mind with visions of bipedal blue-skinned aliens that just happen to be the same size as us?
There are two huge anthro-biases of scale: size and time. A single example can explain both: imagine a life form in a gas giant (like Jupiter), where each individual's diameter is measured in kilometers. For obvious reasons this lifeform will 'run' at a slower pace than something that is our size... it may take hundreds of seconds for a signal to move from one part of its body to the other. The organism, were it intelligent, might experience time at a far slower pace than we do. The time between stimulus and action, e.g., a decision to move away from a threat, might be measured in minutes. Such an organism might seem horribly impractical to you, but from the point of view of the organism - that's just the way time moves. It might view humans as outrageously frenetic; an exaggeration of the way we look at hummingbirds.
A contrasting example comes from Robert Forward's book "Dragon's Egg", where a nano-scale lifeform evolves in a matter of weeks on the surface of a neutron star. Most likely it was this book that originally opened my mind to the idea of truly 'alien' life.
Other biases are easy to spot now: assumptions about gravity, density, temperature, etc. I believe life will arise any place where a certain threshold of complexity is met. In Earth's case, the huge oceans, freely mixed, with an abundance of heavier elements are probably what made the difference. Carbon chemistry has definitely been a big win, but is it really impossible to imagine other kinds of 'organic' chemistry, like Silicon?