While new credit only accounts for 10% of your FICO score, it really can't be ignored. If you're young and don't have a lengthy history, opening up a new account can hurt your score as too much credit is generally considered risky. If you have what might be construed as too many accounts, then the thought is that it's too much to keep up with, especially all those different payment dates.
Although the length of your credit history only weighs in at 15% of your score, it is still vitally important. Don't confuse this with your payment history; your payment history is the quality part, while your payment length is the quantity part.
What Will Happen To My Credit Report?