Before 1968, the units for absolute temperature were described as "degrees Kelvin" or "degrees absolute." After that, the SI system got rid of the idea of "degree" for absolute temperature, so the new unit should apparently be expressed as a "kelvin" (with lowercase k) and abbreviated simply "K" (without the degree sign). Also, official SI conventions suggest that not only should the unit name be lowercase, but it should be pluralized as other units would be: "Il en résulte que la température thermodynamique du point triple de l’eau est égale à 273,16 kelvins exactement, Ttpw = 273,16 K." Or, in English: "It follows that the thermodynamic temperature of the triple point of water is exactly 273.16 kelvins, Ttpw = 273.16 K."
Despite the official SI usage, however, it seems that there are still a variety of conventions in use. Many of the questions on this forum, for example, use Kelvin (with a capital) instead of kelvin in referencing the unit. Also, it appears that the plural usage is somewhat mixed in the physics literature: something like "200 kelvins" occurs, but more rarely than "200 kelvin" or even "200 Kelvin." The NIST guidelines do not list the kelvin as an exception to the normal pluralization rules: "the following plurals are irregular: Singular — lux, hertz, siemens; Plural — lux, hertz, siemens." On the other hand CERN's writing guidelines suggest that there is an exception: "And note that it is always kelvin, even when plural (not kelvins or degrees kelvin)."
Given all of this, here is my question: Is the SI standard actually to pluralize kelvins, as would be suggested from the quotations from the official SI guides above? Is this officially stated anywhere in some standards organization's guidelines? Or, is there some rationale given somewhere for the continued use of plural "kelvin" (as in the CERN guidelines) or even "Kelvin" (with an apparently anomalous capital)?
Or, is it — as I suspect — just a failure to treat the kelvin as an actual SI unit, despite the redefinition from "degrees Kelvin" to "kelvins" that happened decades ago? (Perhaps we just dropped the "degree" but effectively still treat it the same way as Celsius or Fahrenheit?)
EDIT: Just to be clear — I did NOT intend for this to be only a question about linguistic convention. The SI redefinition of the absolute temperature scale and units was apparently meant to refine or change some physical conception about temperature. Resistance to this change may indicate some other elements about the underlying physics (perhaps including the fact that temperature is an intensive property, as suggested in one answer below). While I'd certainly be interested in other official standards and usage recommendations, I also wanted to know if there were other PHYSICAL rationales for the inconsistent units.
Best Answer
The NIST style guide is pretty good — that's a place where people really care about getting details right.
I use lower-case names for spelled-out units, even when named for famous people or having uppercase abbreviations (N -> newton, J -> joule, L -> liter (unless you count $\ell$ for liter), K -> kelvin). I think that "degrees kelvin" is entirely by analogy with "degrees Celsius" and "degrees Fahrenheit" (the latter two of which I think I have always seen capitalized).
I don't pluralize "kelvin" when talking about temperatures: I talk about temperatures like "two hundred fifty millikelvin," or "four kelvin" for the boiling point of helium, or "three hundred kelvin" for room temperature. This is not consistent with the way that I would discuss a length unit, or a mass unit.
I think that this may be because temperature is an intensive variable. If I have a thing that weighs a kilogram, and another thing that weighs four kilograms, and I put them together, I know that I have five kilograms worth of stuff. But if I have some stuff at one kelvin, and some other stuff at four kelvins, and I mix them together, I don't get some stuff at five kelvins. I know this, and so I don't think of "a kelvin" as a lump of temperature that I can carry around and add or subtract to things.
I feel the same way about the hertz: I have no desire to say "sixty hertzes." Combining an oscillator at 60 Hz and an oscillator at 10 Hz gives me something much more complicated than an oscillator at 70 Hz. I notice that "hertz" is listed as one of your three exceptions, though.
If that's really my thought process I would make the same decision about the pascal (for pressure) and the poise (for viscosity); I can't think off the top of my head of another intensive quantity with a named unit. I think that if you asked me I would tell you that air pressure at sea level is "ten to the five pascal," but I'm focusing too hard on it and I'm honestly not sure.
In response to a comment: I definitely do say things like "two atmospheres of pressure," but never "two bars" or "one thousand torrs." It could well be that dealing with kelvins one by one is so rare that I don't think of them as being countable. Interesting.