Rollover, ghosting, and keyblocking
If you have heard enough gaming keyboard spiel from manufacturers, you probably have come across jargon such as anti-ghosting, key rollover and key blocking. Key blocking is the tendency of the keyboard to refrain from registering any input when multiple keys are held down simultaneously. The key rollover figure is closely related to this phenomenon. You will generally see it advertised as X-key rollover, where X is the number of keys that can be pressed simultaneously before key blocking occurs.
It is of utmost importance to gamers because certain types of games such as stealth-based FPS (sneaker shooters) require multiple modifiers for crouching, running, leaning and zooming being held down even as you press more keys. Therefore, a keyboard with 6-key rollover will let you hold down a lot more keys than one with 2-key rollover (4KRO), while also allowing you to use the WASD cluster as usual. Although some PS/2 keyboards let you hold any number of keys down (NKRO: n-key rollover), certain USB gaming keyboards also boast of that capability. However, it's very rare that one would require any more than 6KRO.
Ghosting occurs when you press two or more keys and the keyboard mistakenly register an extra key that wasn't pressed at all. This is caused by keyboard design that determines key input by shorting entire rows and columns of the keyboard matrix. Fortunately, this issue is no longer observed in modern keyboards. Among these parameters, rollover is the only one that deserves to be considered then. At any rate, you can test your keyboard's rollover and ghosting capability with this handy tool.
N-key rollover makes a gread deal of difference in stealth FPS games
Is a 1000Hz (1ms) polling rate relevant?
A polling rate figure of 1000Hz (1ms) is commonly touted by most gaming keyboard manufacturers. But is it really required? If you consider the fact that (as explained earlier) Cherry MX mechanical switches require an anti-bouncing delay of 5ms, isn't this 1ms "ultrapolling" advantage rather moot? Well, that surprisingly isn't the case. Yes, it's rendered irrelevant by the 5ms anti-bouncing delay, but only if we consider double taps. You see, the first key press or multiple actuations of different keys aren't limited by the anti-bouncing delay. In these circumstances, a lower 1ms latency can actually make a difference in games that require precise timing.
Still don't believe me? Allow me to illustrate with an example. Consider an ultra-twitchy game such as Super Meat Boy, where the sprite travels an onscreen distance of, say, 2000 pixels in a second. In this case, a quick polling rate of 1000Hz (1ms) will let you jump after every 2 pixels. However, the default USB polling rate of 125Hz will register an input only once every 16 pixels. If you head over to MS Paint and measure 16 pixels, you'll realise that's a lot of latency. In fact, it's a bit wider than Meat Boy himself! However, Meat Boy in reality runs at half the speed, which still makes a world of difference in a game as precise as that. In the very difficult and timing-dependent stages of the game, I personally have seen a difference with 1000Hz keyboards.
Other bells and whistles
With the important bits taken care of, it's time to take a look at the frills that most gaming keyboards are equipped with to justify their premium price tag. No gaming keyboard is complete without a driver suite that lets you customise key assignments, set profiles, macros and more. It also pays to have at least one column of macro/programmable keys next to the WASD cluster. This is of utmost importance for MMO/RPG gamers, who benefit from binding complex tasks to such programmable keys. Since gamers are no strangers to travelling with their gear to BYOC LAN parties, having inbuilt memory on the keyboard saves you from having to setup the whole shebang on a different computer.
Backlighting comes in handy for noctural gaming sessions
For the price, it also doesn't hurt if the manufacturer throws in a wrist rest. Gaming is largely a nocturnal activity, so it's a good idea for a keyboard to be backlit. Mechanical keyboards with individually backlit switches are always better than those using edge-lit lighting clusters. A few extra media control buttons can make listening to music and watching movies free from the travails of fumbling with the GUI. An analogue jog dial for volume control is any day better than half-assedly co-opting F1 and F2 keys for the purpose with the help of a modifier key.
Well, that just about covers all that you need to look for when choosing a gaming keyboard. Just make sure you try out different types mechanical switches before zeroing in on the perfect one for you. Good hunting!