What are the minimum set of physical characteristics to define an Earth like planet?

Introduction

I have a vague notion of building some sort of strategic space game. Similar to other strategic games, locations are assigned value based upon some abstract evaluation. Following in that vein, I'd like to assign value(s) to planets based upon a number of factors including colonization.

If you feel that this particular question can be answered in two (or more) ways depending upon what I mean by "Earth like", then for this question assume I mean for purposes of human habitation. If you can easily slip in other measures of value, feel free to do so but be sure to distinguish how this value is different than that for colonization.

Setting

Humans have figured out some mechanism for traveling to other star systems. When we get to these new destinations we want to evaluate the "value" of the planets. One basis for this valuation will be colonization ease (how easy to transplant humans & human life to the planet) and potential (what is the total population this planet is likely to be able to support).

Likely Measures

Measures I think are likely to be crucial:

• Surface Gravity (higher than 1 g is worse than lower than 1 g)
• Surface Pressure (higher than STP is worse than lower than STP)
• Surface Temperature (higher than STP is worse than lower than STP)
• Surface Radiation (high bad, low good)

Of course matching atmosphere to what we need would be good but as a starter planet that we can terraform, we might be able to begin with a planet with Earth normal atmospheric pressure. Humans would just require oxygen masks as long as the atmosphere wasn't corrosive or toxic.

Can you think of any other valuation criteria?

Has some other scientific body considered this question already? If so, what did they think?

Also, I like the original Traveller RPG. It had a hex scoring of a planet based upon a variety of factors (gravity, temperature, pressure, radiation, etc.) but that valuation works better on a scalar value like pressure than it does for something as diverse as atmosphere.

I was thinking of doing something like a "snow line" scale for my linear scale and then some other marker if the atmospheric composition deviates from the norm.

Snow line - planet composition chart