"Less" appears to belong to the category called quantifiers, {some, many, all, ...}. Semantically these words express quantity and syntactically they often exclude articles from their normal position at the front of a noun phrase. Thus one says "all students," and not "the all students" (though "all the students" is also possible) and "every student" rather than "an every student."
Since "student" is a count noun, and since "less" (as well as "more") goes with mass nouns ("some" goes with either mass or count), and since mass nouns do not take an article anyway, it must be admitted that the above syntactic argument loses its force. A perhaps better example is the quantifier "no," which excludes articles with both count and mass nouns: "(he has) no ticket, (so he gets) no laundry."
Note that "a more interesting argument" is not a counter-example, because this instance of "more" is part of the adjective phrase, "more interesting." (At least semantically it is clear that "more" does not apply to "argument.")
Also note that superlatives (with count nouns) restrict the referent (what is being referred to) to a single entity (not necessarily physical) and so take an article which must be the definite, "the," as in "Alex asks the most questions."
For count nouns, "fewer" is preferred to "less." Thus one says "Jane has fewer marbles than Mary, but Mary has less gum." Both words appear to be comparative forms - of "few" and "little" respectively. Thus one can say, corresponding to the above, "Jane has few marbles, but Mary has little gum." The latter uses "few" and "little" in what might be called an upper-limit sense, as opposed to the lower-limit sense that one gets when using an (indefinite) article, as in "Jane has a few marbles, and Mary has a little gum." (Also note the shift in conjunctions, from "but" to "and.")
If "less" is a comparative, what can we possibly say about "lesser"?! Well, first note - in support of the position that "less" is already a comparative even without an "-er" - that there is a strong parallel between "less (than)" and "greater (than)." Syntactically they can be used in the same places and semantically they are identical except for a reversal of polarity. Also note the rather peculiar fact that where "lesser" appears (e.g., "a lesser god") it is not possible to substitute "less." Nor is it possible to use a superlative in that slot.
Here is a summary of some amount words, each with its base form, comparative and superlative: