The simplest answer is Complex Chemistry. You and I can be thought of as bags of very complicated self-sustaining chemical reactions.
To that end, we need:
- Lots of different chemicals: a ball of hydrogen gas isn't going to
do much on it's own.
- Density: chemical reactions happen faster when the reactants are densely packed. Liquids work great for this, but so might a gas-giant. A planet with solid, unchanging rock probably wouldn't do much.
- Outside source of energy: Energy-input would allow more interesting chemical-reactions to take place. More specifically, sustainable reactions to occur.
- Stability, but not too much stability: If a planet goes from 1000 K to 20K each day, it's going to be hard for life to form. Conversely, if a system is totally stable, there isn't going to be enough 'mixing' to get metabolism going.
The search for habitable planets, and for possible extra-terrestrial life-bearing planets is one fraught with HUGE amounts of guesswork. We have no real clue what alien life might look like, so we focus on looking for stuff similar to our own earth. So, things like:
- Habitable Zone - the planet needs to be in the correct distance to maintain the right temperatures for liquid water to be present
- Magnetic Field - Something to shield life from intense radiation
- Presence of water - Again, we think life originated in water, so searches for life tend to focus on this.
Here's some fun food-for-thought:
The elements needed for carbon-based life: Carbon, oxygen, nitrogen, hydrogen - are all going to be very common on high-metallicity star-systems. They are readily formed by stellar neuclsynthesis, and -should- be present on most population I and II stars (have a read-through of : http://en.wikipedia.org/wiki/Metallicity)