The 1921-D is the lowest mintage Walking Liberty half dollar and it is very rare in low grades. It is also the most expensive Walker in the very lowest grades. The key date 1921-S has a higher mintage and is more available in grades VG and below, but the 1921-S "catches up" in higher circulated grades and is rightfully more expensive than the 1921-D in grades VF and higher. Finding a 1921-D in any circulated grade is very difficult. In mint state grades, the 1921-D "tied" for third rarest issue of the series with the 1919-S as only the 1921-S and 1919-D are rarer. Apparently a few Gems were saved as you occasionally see a really nice specimen. In Gem condition, the 1921-D is about as rare as the 1917-S Obverse, 1918-D and 1918-S, with the 1921-S and 1919-D being the major rarities of the series in Gem condition.
Mint state 1921-D half dollars, when encountered, are usually pretty nice. Strike is normally not a problem and full detail is the norm. The luster can be soft satiny to quite frosty. Color ranges from full white to deeply toned.