What are some things to consider when choosing a salt?

Most of the issues come down to the additives and the shape of the salt.

Some people find that iodized salt gives some off flavors, while sea salts will have regional differences in their mineral content that affects the flavor.

In my mind, the bigger issue is the shape of the salt:

• Kosher / flake salt : See Darin's comments for examples of some of the benefits.
• Pretzel salt : large crystals, gives a crunch when eating it.
• Popcorn & Pickling salt : fine crystals; will disolve in room-temp water and stick well to relatively dry items. Pickling salt is uniodized.
• Rock salt : good for salting paths and salting ice to bring down the temp when churning ice cream.

For general baking, or salting water for cooking, I tend to stick with cheap, old fashioned table salt, in part so I don't have to convert recipes. And I use iodized, because I don't eat out much, or eat much seafood or processed foods.

For general cooking, I tend to use kosher salt, mostly because I keep it in a container I can easily get a pinch of.

I've never bothered with most gourmet sea salts ... maybe I'm not using enough, but other than smoked salts, I don't really taste the difference.

I would not recommend plain table salt for any application. Kosher salt makes a good general purpose cooking salt. Keep it in a bowl by your stove and use it when you need a pinch or a teaspoon. The larger grains make it easy to handle. There are also sea salts appropriate to this purpose, though they are more expensive.

Most other sea salts, such as Maldon, sel gris, fleur de sel, and hundreds of others are meant as finishing salts. A small sprinkling of these beautiful salts on a finished dish, either by the cook or at the table, can makes an enormously satisfying experience.

Although the flavors of plain sea salts are fairly similar, the colors and texture vary greatly. This is where the artistry of pairing salt to food can come into play. If you are looking for a good place to start, I highly recommend Maldon sea salt, from Great Britain. It has a flaky, crunchy texture that is easy to love and usable on a huge range of foods. It is well regarded by virtually every cook and chef.

Then there is also a range of flavored salts; generally I don't find these as useful as I can add my own flavors to a dish. One exception is some of the smoked salts, which can provide a little smoky hit that is quite appealing.

An excellent resource for learning about salt is At The Meadow, a shop in Portland that has an excellent website. Their owner, Mark Bitterman, is releasing a book about his lifelong search for amazing salts. (Disclaimer, I had a brief affiliate relationship with them, which is no longer active).

For me there are three considerations:

1- Handling

This one is the most important. Kosher salt can be picked up easily and pickling salt dissolves well and sticks to popcorn.

2- Flavor

This is really only a factor for fancy sea salts with other minerals or smoked salt. This really only matters to me for salts at the table.

3- Iodine

Not much food in my diet contains iodine. I sometimes use iodinated salt- especially in baking- just to make sure we get some. Getting goiter has become so unfashionable this century.

Overall I use kosher salt far more than any other.