"Coins that are considered scarce due to low mintage or low surviving specimens for that date (and possibly mint mark). Prices are higher on scarce issues, and they are harder to find." coinresource.com
Quote above is self-explanatory. Key date coins is a must for every collector especially for those who are into coins. It's a real please if you can complete a series with their respective keys. Even our current circulating coins (in your pocket!) has its own key dates.
I don't know about other collectors, but every coins that fell into my hands were never had a chance to escape my strict checking. What if I stumble upon a 10 cent 1971? Absolutely it will be among the happiest day in my collecting life.
Why key date coins worth more than ordinary coins? The most obvious reason is that they are simply limited in number compared to other dates, hence harder to be found. Let say you have a 200 pieces of Malaysia 50 Sen, if you are lucky enough, maybe you will found only one Malaysia 50 Sen dated 1995. Try it if you don't believe me.
Get excited? Okay this is the list for you to look for in your coins. If you are not a collector, but happen to have these coins, do inform me so that I can publish it (with the photo and your name in it) in my blog. Cheers!
1. 50 Sen Malaysia - 1969, 1995
2. 20 Sen Malaysia - 1970, 1978, 1994
3. 10 Sen Malaysia - 1971, 1994, 2003
footnote: I will update later a proper table for list of key dates coins.
Appendix: photo of some of my recently discovered key date coins.