What are the criteria that make a planet (in)habitable?
Circumstellar habitable zone

There are a few pages that explain the concept in detail, but the gist is that the circumstellar habitable zone is (my definition)


  The region around a star inside which a planet similar to Earth can have liquid water on it surface.


However, with more detailed observations of Europa, with its possible underground ocean, and Titan, with its hydrocarbon lakes, the definition may be modified:


  The region around a star inside which a body can have liquid water, or where other compounds suited for the formation of life can arise; alternatively, the region around a gas giant where moons can be heated by tidal forces such that the compounds can exist.


The latter section, of course applies to moons only. The galactic habitable zone (see here and here) is another - slightly less-well-defined - area where a habitable planet should lie.

You can calculate the habitable zone around a star based off of what defines it: the luminosity of the star, primarily. There's a poorly-explained formula here, although Planetary Biology gives a much more explicit derivation. The two important equations are these:
$$r_i = \sqrt{\frac{L_{\text{star}}}{1.1}}$$
$$r_o = \sqrt{\frac{L_{\text{star}}}{0.53}}$$
where $r_i$ and $r_o$ are the inner and outer radii, and $L_{\text{star}}$ is the luminosity of the star. Note, though, that these are for the conventional definition of the habitable zone, neglecting non-$\ce{H2O}$-based compounds and tidal heating of moons orbiting gas giants. This pre-print, by Kopparapu et al., does give another interesting formula:
$$d=\left(\frac{L/L_{\odot}}{S_{eff}} \right)^{0.5} \text{ AU}$$
where $S_{eff}$ is a parameter determined by the effective temperature $T_{eff}$ of a planet at that distance and some coefficients, as well as the Sun's $S_{eff}$. But those findings are rather new, so I'd stick with the older formulas.

So the habitable zone is determined primarily by the star's luminosity.

Mass

An atmosphere is generally considered a must for planets with Earth-like lifeforms. Low-mass bodies, such as the Moon, can't hold on to one, and that's one of the reasons that moons have not been of as much interest as planets have been. Atmospheres, among other things, can keep the planet at a nice temperature and allow distinct climates to form. As this site elaborates on, they also help protect the planet from radiation like UV rays. Our ozone layer is really helpful in that regard.

Mass isn't the only thing that helps a body keep its atmosphere. For example, Titan is relatively low-mass (although relatively high-mass for moons), yet it still has an atmosphere. This is because the solar wind - which can hurt atmospheres - is so weak at Titan's distance from the Sun.

To expand on what gansub mentioned: Magnetic fields are important because they are extremely useful when it comes to helping a planet retain its atmosphere. As Luhmann and Russell explain in "Mars: Magnetic Field and Magnetosphere", Mars lost its magnetic field long ago, and so the solar wind is gradually stripping it away, albeit at a really slow rate.

Rotation

This section is based a lot on this paper, by Yang et al. It, too is recent, so keep that in mind.

Before I get into the paper, I'll say this: Rotation helps because it keeps one side of the planet from being baked while the other side freezes. Tidally-locked planets aren't the greatest places for life. A decently-fast rotation can really help.

Anyway, Yang et al. came to this conclusion (as voiced in their opening sentence):


  Planetary rotation rate is a key parameter in determining atmospheric circulation and hence the spatial pattern of clouds. Since clouds can exert a dominant control on planetary radiation balance, rotation rate could be critical for determining mean planetary climate.


In other words, rotation helps clouds, and since clouds help govern the climate of the planet, a proper rotation rate can make the climate less extreme.