Eating food that is most appropriate to where it is grown makes the most environmental sense. A “local” veggie may be more environmentally harmful than one trucked in from elsewhere because the “foreign” veggie may have conserved resource (water, soil health, etc.) well out of proportion to the resource used to transport it.
Most foods require far more hydrocarbons to grow than are used in transport… in addition to mechanized production (tractors, irrigation pumps), nitrogenous fertilizer production requires either natural gas or utilizes manure, the production for which is even more carbon-intensive. Much produce is grown with heavy irrigation that depletes the local water table permanently. This is one of the major arguments against putting corn-based Ethanol in gas; that not only does it produce more CO2 in the end, it doesn’t even save petrochemicals, and is a waste of water besides…
Would you like an example of harmful local produce? If you’ve ever taken a plane trip over the Southwest, you can see all the obviously heavily-irrigated fields right on the edge of the desert. (It’s obvious because they are surrounded by completely-bare brown earth.) And you know that soil ain’t exactly fertile without some help. Heck, few “local” veggies from the salad bowl of America, CA’s inland valley, would exist at all without heavy irrigation.
As far as most of your other reasons? Those are personal cultural decisions that won’t apply to most people. (Speaking for myself, I’ve never felt the need to personally “investigate” how my food is made (brings to mind “Colin the chicken” from the Portlandia pilot.) Nor do I think a local farmer is somehow more deserving of my business than one a couple of states over if the other farmer has managed to produce a superior product for a good price. Eating foods popular in the local cuisine may or may not involve locally-produced products.
I don’t consider somebody who prefers to eat in-season produce to be a “locavore”; for highly-perishable items (especially delicate fruits and vegetables), a desire for the highest-quality items will generally lead you to closer stuff without consciously choosing “local” items.
Lastly, it’s very easy for somebody in CA (or maybe the gulf coast) to proudly proclaim their “locavore”-ism; those of us in the rest of the country that like fresh food and have no great desire to eat root veggies all winter long must resign ourselves to our (what’s the opposite of locavore-ism?) status.