I completely agree that you should start saving money for retirement and slow down your mortgage payoff. Although it is an emotional issue having that huge debt hanging over your head, paying off your mortgage at 2.8% is not in your financial favor (over saving the money. It is better than spending the extra cash).
1. 2.8% is around inflation. Do you want to pay off the mortgage in today's dollars, or in next year's dollars that are worth 3% less?
2. There is a tax benefit to paying mortgage interest of whatever your marginal tax rate is. If you are in the 25% tax bracket and itemizing, then you save 25% off that interest. Now you are only paying around 2.1% interest.
3. By investing in the stock market, you should get between 7-10% over time. That is, at a minimum, more than twice as much as you are saving by paying off the mortgage early.
In short, over the course of the next 15 years, you would be better off putting that money into a tax preferential investment account like a Roth IRA or a 401k, or even just a standard brokerage account. You will have more money in the long run.
I'll run some numbers for you.
Assuming a $200,000 mortgage 15 year @ 2.8% your payment is 1362.01 beginning in Jan 2013.
If you have an extra $1000/month to pay it off, you will pay it off in Nov. 2020. Then invest the full amount 2362.01@7% until Dec. 2027 and you will have $259,717.
If, on the other hand, you just invest $1000/month starting now for 15 years, you will have $316,962 when your mortgage is paid off.
This doesn't even factor in the tax savings from the mortgage interest deduction.