I'm a software developer who spends all day on a keyboard, and I'm currently using a Microsoft Natural Ergonomic 4000 (the split keyboard helps reduce/eliminate wrist pain). What's better/worse/different about a mechanical keyboard? Enlighten me since I'm too lazy to just Google it.
It's a blend of differences in tactile feedback and response.
In a mechanical switch, the action of the mechanism that makes the "contact" creates a physical and/or audible sensation during the press. Because of this, your finger "knows" when and where (along the stroke) the key was pressed - that is, exactly when the keypress signal was triggered. A softswitch made with a membrane, in comparison, feels 'mushy' and less 'certain,' because there's no response at the time the actual signal is sensed. The key still travels, but you can't "tell" where along the travel the keypress signal happens.
This matters for fine timing (in the case of a gamer trying to act at a certain time), but it also creates a sense of "sureness" in the typing, as well. This is a big part of what people mean when they say they "feel" better.
Also, some softswitch keyboards react strangely to things like pressing multiple keys at once; mechanicals usually resolve all the keypresses properly, but that's not because they're mechanical, that's because makers of (pricier) mechanicals are less likely to us cheap electronics.
Mechanicals also generally have better springs and often longer key travel; this does allow for faster and better-controlled repetition. This only matters for gamers and very fast typers. The better rebound may reduce strain for the typist, depending on their technique - striking a key that rebounds your finger is less tiring over time than pushing mushy buttons and having to pick your fingers back up, if you're used to striking the keys, as opposed to touching them.