Well, there's an angle I haven't heard mentioned on this site before (been lurking for a few years): if you're making extra principal payments on your mortgage, you're likely doing it with income that's been taxed - so in order to do that, you're paying FICA tax and marginal tax rate for the privilege of making extra principal payments. In contrast, if you're instead investing in stocks with that same money, you *could* be doing it solely in tax deferred accounts: HSA that avoids FICA and marginal tax rate; 401K/403B/457 that defers taxes... but remember, you're not just deferring those taxes to a later date: if you do it right, you could end up paying 0% on that income if you stay in the 15% bracket in retirement.
So folks can talk all they want about how paying down the mortgage gives them guaranteed returns and peace of mind, but they are also guaranteeing that they will pay more in taxes for that guaranteed return.