I agree it's a very personal decision. The main 2 options are to pay it off or somehow invest the money at a higher rate, thereby earning more on the investment, and retaining the tax credit. It's hard to base it on numbers because the numbers involve a certain amount of guestimation/ fortune telling. It's easy to make it look better to keep the mortgage on paper. But that involves you having knowledge where to invest it, and having the will to invest it without taking it back out.
Personally my goal is to be debt-free including the mortgage. For me it doesn't matter what percentage the past investments have paid at...the economy is still fragile and the future is unknown. Also, I don't know that I would have the will power to give that much money to the whims of the market, however I would have no problem doing it for a guaranteed return of 5.375 (my mortgage interest rate). And it's more difficult to get cash out of house than it is to sell some stock; some might say this is precisely why you shouldn't that much money into the house, but I say that is a good thing.
My plan is to pay the mortgage off very quickly (by year 8 of the mortgage) AND then start investing the savings. Automatic 40k savings, and my money is freed up for investment ventures.
However, as long as the tax law is that favorable for you, even I can't argue much with a 1.75% rate. There are CDs going for more than that. It depends on what your personal goals, and the type of effort you want to put in.