As a paramedic, the information that would be useful to me includes:

Name and Date of Birth
Medical conditions that could be life threatening
Severe allergies, especially to medication
Certain medications like anticoagulants (blood thinners) or beta-blockers
Contact information for someone familiar with your medical history and who can make medical decisions on your behalf if you are unconscious
Donor status (though an ID band is not a legal document allowing harvesting of organs, it at least indicates your wishes)

There is no need to list your blood type. If you're seriously injured the first blood you will receive in any hospital is O neg, commonly referred to as trauma blood, which can be given to anyone. If your condition is less immediate, they will draw and cross match before giving you any blood. No medical facility would ever give type specific blood based on an ID bracelet. If you are going to use any abbreviations to save space, make sure you know the approved medical abbreviations.

Here is an example of an ID band that I would find useful:

John Q Cyclist DOB 4-13-1972
CAD, a-fib, Type 2 diabetes
Medications: Coumadin, Metformin, Metoprolol
Allergic to sulfa, PCN
ICE: Maria L Cyclist 444-555-6666 cell
Organ Donor

Of course how much information you can get on your ID depends on the brand and style. It is always a good idea to carry a picture ID with your name, DOB, and current address. With modern electronic medical records, hospitals can usually find your information quickly if they have proper means of identification.