As noticed in my pages and numerous others, there is a significance placed on the blood type of anime characters. In just about every anime book you can find a section for every character telling their blood type, as it signifies characteristics of that person. After some searching on the net etc., these seem to be the most common explanations of the significance of blood typing (thanks to sushi for Animazement's descriptions):

Type A

Advantages: obedient, careful, sympathetic, empathetic, self-sacrificing, polite, willing to compromise, honest, loyal

Disadvantages: worries a lot, emotional, weak-willed, indecisive, introverted, anti-social, wishy-washy, nervous

Type B

Advantages: cheerful, outgoing, optimistic, adventurous, active, sensitive, kind

Disadvantages: forgetful, undecided, disorganized, noisy, spontaneous, prone to exaggeration

Type AB

Advantages: sensitive, proud, diplomatic, discriminating, easy-going, sympathetic, efficient, learns quickly

Disadvantages: short-tempered, complains, dependent, moody, brooding

Type O

Advantages: confident, strong-willed, judgemental, dedicated, self-deterministic

Disadvantages: workaholic, insecure, emotional, stubborn, uncompromising, cold personality, over-confident, self-centered

Well, it's obvious which blood type is most popular, huh? Very interesting.