In my experience, condition rarity key dates are a largely untapped segment of the coin collecting market. A condition rarity is a coin that is generally common, but becomes extremely tough to find in higher grades. A well known example is the 1944 red cent. It can be quite easily acquired in all grades, but any example with full red color is extremely difficult to acquire, and will fetch significant prices. Many condition rarity coins are not well reported, which can create a gap between the price justified by availability and the actual market price. This may not always be the case, but there are bargains to be had if you know where to look.
What follows is a list of condition rarities I'm familiar with (which is admittedly not a comprehensive list). If you know of any others, please send them my way and I will add them to the list. An important note is that some of the rarities listed here may cease to be rarities in a few years. The 1989 5 cent in MS-65 is a rarity now, but if a few more examples come to market, prices should be expected to drop. I'm not saying that will happen, but it's always a possibility -- such a drop is more likely to occur with modern coins than with classics.
A few curious coins.
1970 5 cents
1970 10 cents
1991 25 cents
These are interesting because they are the opposite of a condition rarity. They are almost always found in Mint-state condition, but are next to impossible to discover in lower circulated grades. These coins were recognized for their low mintages in the year of issue and were extensively hoarded. Very few ever spent any significant time in circulation. Worn examples of these coins may be an interesting conversation starter, but they do not command a premium price; it doesn't take much effort to take a MS example and bring it down a few grades.
True condition rarities
All Newfoundland red small cents. Low grades can be picked up for a buck or two, but you hardly see MS-64 or higher examples on eBay.
1953 SF 1 cent, MS-65 full red.
1983 1 cent in MS-65.
1990 cent in MS-65 red or better. It was a lower mintage year, but with a mintage high enough that people didn't bother to hoard, a common element in these condition rarity coins.
MS-65 10 cent, 25 cent and $1 coins from 1968-present. I am painting with a broad brush here, but there is a clear division in Canadian coin collecting. Generally speaking, Canadian coins can be divided into the silver era from 1858-1967, and the modern era. Many collectors stop at 1967, and the nickel examples from later years are often tossed into junk piles or spent. Couple that with the tendency for Nickel coins to pick up bagmarks easily, and the nickel series becomes very tough to find in the highest Mint State grades.
1959-61 Silver dollars in MS-64. These were largely considered common in comparison to the earlier silver dollars, and were given the bulk bucket treatment by dealers for many decades. Finding one without a baggy surface is a challenge.
1965 Type 2 and Type 4 in MS-64. The Type 5 gets all the hype, but it's probably easier to find in MS-64 condition than these two examples. Again they are another "common" coin that has been tossed aside for years leaving very few survivors in nice condition.
Many collectors will claim that these coins are very common, and that it hasn't been worthwhile for people with hoards of these coins to have them certified. I have trouble believing this statement given the prices many of these coins achieve on eBay when they appear. While some high grade examples are undoubtedly still out there, I believe that many of these coins will ultimately be seen as genuine rarities.
Thanks again for taking the time to read my guide. If you've enjoyed it, be sure to read my other guides, and click "Yes" below to let me know you've found it helpful. Don't hesitate to forward any examples you know of, and I'll add them to the list. These coins can be both challenges, as well as bargains, best of luck and happy hunting.