1) If you don't pay off your balance each month, rewards credit cards probably aren't worth it in the long run
2) Many cards impose limits on what you can earn – especially those with higher earn rates

The other thing I'd add is that many cards come with an annual fee and you should factor that in when you're calculating what you'll earn in a year.

It used to be that you had to use a credit card to get meaningful rewards but many people would rather use a debit card. My name is Laurie McLachlan and I work for PerkStreet Financial. We're introducing an online checking account with a debit card that gives 1% back in the form of cash back or free music or free coffee. Just another option to consider.

http://www.perkstreet.com