clarry: I don't buy the argument about mechanical keyboards having lower response time. Electrically, there is no need for it to be so. It all boils down to how the controller is programmed. Likewise, there's no reason why a rubber dome key should not last as long as or longer than a mechanical switch. It's a very solid construct, well protected with not much moving parts, and in my experience suffers less from oxidation (which can be an issue with mechanical switches). I have never had a rubber dome keyboard break due to age or excess use, but I have mechanical cherry mx switches that are having contact issues, especially if left unused for some time.

The main difference between a mechanical and rubber dome keyboard is that a rubber dome makes contact -- and signals a key press -- only when you "bottom out." It's not very comfortable for fingers to bottom out, but you might not notice if you're used to it. Mechanical keyswitches generally conduct at a point partway into its travel, and switch design can incorporate a tactile bump at the activation point. You'll also find very linear and very clicky switches.

Why would you not want one? Pretty much price is the only reason I can imagine. Many switches are loud, but I hear there are quieter switches around, and people also mod them with O rings that are supposed to dampen the clackity clack.

Also, choosing the best switch for you might be easier said than done. And IMO not all switches are good. So you might spend more money on swapping switches.

EDIT: attached photo of my current keyboard. It's got kailh box white switches, and it's loud. 200 usd + tax & customs. One day I will build my own keyboard.

dtgreene: By pressing the key slowly, I determine that bottoming-out is not necessary; a keystroke is made right before the key reaches its bottom position.

So, how is a mechanical keyboard different from the keyboard I already has? Or is my current keyboard basicallt a mechanical keyboard?

duallaser: "Mechanical keyboard" is too ambiguous of a term.

clarry: rubber domes with NKRO.. but usually cheap keyboards aren't NKRO because you need a diode for each key.

I don't see an issue with gaming on clicky switches. And I'm not a fan of linear switches myself.

idbeholdME: There are but they are very few and far between and the usual ones you see everywhere are not. And if you are going to invest into a better keyboard, you might as well get a mechanical. The key for NKRO is using the PS/2 connector. USB is simply incapable of that.

clarry: Not true at all, I have USB keyboard with NKRO. They are common.

idbeholdME: Then they are using some proprietary communication protocol instead of the usual USB.

clarry: Yes, they are sticking all the keys into a bitfield instead of stuffing modifiers in a bitfield folllowed by a short array of keycodes. It's a de-facto standard and modern operating systems support this out of the box. (For example, OpenBSD got support 7 years ago: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/dev/usb/Attic/hidkbd.c.diff?r1=1.5&r2=1.6)

Here's the source code for the report descriptor used by my keyboard: https://github.com/qmk/qmk_firmware/blob/master/tmk_core/protocol/usb_descriptor.c#L192

idbeholdME: EDIT:
So there apparently is a full-speed USB HID mode which can allow NKRO, meaning keyboards which are made to utilize it can do NKRO over USB. But it isn't used often and usually results in the keyboard not showing up in POST and not working in BIOS. So it is possible but unless the manufacturer tampers with this, the usual 6 key rollover limitation from the USB stands.

idbeholdME: The key for NKRO is using the PS/2 connector. USB is simply incapable of that.