Does the song fit with the others? Are the genres of music the same or at least similar? Most people like certain types of music and can be turned off by dramatic changes in the flavor of music they listen to. Create a play list of preproduction versions of song and switch between them to get a feel for how they work together before spending too much time recording them.
It might be a great song but it should show case your strengths or you will be selling yourself short. There are lots of great songs out there. Don't waste your time on song that don't make you sound good. If you are recording original material, the same rule applies: make sure it shows off what you can do.
Can the recording be completed within a reasonable amount of time? If a song is taking too long, consider dropping it and picking another song that can be recorded within your available recording schedule. If you are recording a whole album, you don't want to waste your whole budget on one song and have the rest of the project sound like crap.
A CD with 12 songs in the same key would be torture to listen to. Key and tempo are important elements in building the flow of a CD. You can have multiple songs in the same key but you might not want to have more than a couple in a row. See the Song Key article for more info.
If you are making a CD you expect people to listen to in the car, you don't want to include soft songs or songs with a very wide dynamic range. Half the sound will be lost to road rumble. The flow of a dance CD would be damaged by a soft acoustic number. Conversely, a techno song would sound out of place on an acoustic CD.
You'll need to promote your CD so you'll want to be able to perform all, or most, of the songs live if at all possible. Consider holding off on big production numbers till a later release when you can afford a bigger live production team.