Why do sun-dried tomatoes taste different than fresh tomatoes?
There are a number of reasons why the flavor of tomatoes changes during both the cooking and drying processes.

The first is that when drying the tomatoes, farmers and processors will dust the tomatoes in fairly high levels of salt, which helps to keep harmful microbes and insects from eating into the fruit and causing rot and infections.

The second is that when drying anything, you need to remove the water from the flesh of whatever it is you're drying. This causes all of the flavor molecules to become more concentrated. The resulting flavor is more intense because there is simply more flavor per bite.

The final and probably most important (and beautiful:) reason is chemistry. Tomatoes play house to something like 400 volatile aromatic compounds - that is, molecular structures that change and break apart during the acts of heating or drying. While I don't have the EXACT science in front of me right now (and can't seem to find it on the interblags) there's strong indications that it has quite a bit to do with the structure of sulfur-heavy amino acids, c6 volatiles, and glutamic acid breaking down over the course of the drying process - due to the evaporation of water and introduction of salt - and changing into different aroma molecules.

All of these, taken together together have a profound impact on the flavor profile of sundried tomatoes.