Glycerin can cause irritation and burning in women who are sensitive to it, and in some cases lead to yeast infections. It also can make lube feel sticky, especially as it dries, which many people find uncomfortable.
Parabens - if I'm recalling correctly - mimic estrogen, which can throw a woman's normal levels off-balance, and in some cases cause irritation. There's also some speculation (though as far as I know no concrete evidence) that parabens can increase your risk for breast cancer. Again, I might not be remembering this correctly, so if no one else steps in to explain you may want to do some independent research.
How important it is to avoid these ingredients is up to you. If you're more sensitive or more health-conscious, it's probably worth it to avoid them.