I’m sure somewhere here in the Straight Dope this question has been addressed: Is there a difference between freash and frozen vegetables? By this I mean to they taste drasically different and is the nutritional value effected.
I was told by my grandmother that frozen veggies are best because they are frozen right from the field, while “fresh” veggies could be a week to 10 days old. My spouse thinks that unfrozen vegetables are the best.
During the freezing process, ice crystals form, which destroy cells and change the texture of the food. I doubt this affects nutritional value, but it certainly does make them taste different.
Taste is different if you have a sensitive palette (fresh vegetables have a, well, “fresh” taste to them), and as Alereone mentioned, texture is definitely different (frozen fruits & vegetables will tend to have a more soggy consistency when thawed / cooked). There generally isn’t any real difference in nutritional value (cite), so the difference is mainly an aesthetic one. Your grandmother was right though; “fresh” produce sometimes isn’t so fresh and may have a lower nutritional value if it’s been around a while.