A laudatory expression for “an expert in grammar”

Solution 1:

A term that has laudatory connotations is language maven. Wikipedia defines maven as follows:

A maven (also mavin) is a trusted expert in a particular field, who seeks to pass knowledge on to others. The word maven comes from Hebrew, meaning "one who understands", based on an accumulation of knowledge.

As Wikipedia further notes:

Since the 1980s it (maven) has become more common since William Safire adopted it to describe himself as "the language maven". The word is mainly confined to American English, but did not appear with the publication of the 1976 edition of Webster's Third New International Dictionary; it is, however, included in the Oxford English Dictionary second edition (1989) and the American Heritage Dictionary fourth edition (2000). Numerous individuals and entities now affix maven or mavin to assert their expertise in a particular area.

The following is an extract from The New York Times, the newspaper in which Safire wrote a regular column about language issues (called On Language) until his death in 2009:

Though he jokingly anointed himself “Usage Dictator,” he was never comfortable with expectations that he play the part of language absolutist, handing down infallible decrees. “Maven” was a title more to his liking, a Yiddishism that he said contained its own “note of self-mockery.” (An incurable paronomasiac, he also enjoyed punning off the word, as in his 1993 collection of columns, “Quoth the Maven.”)

And further:

As battles raged between prescriptive grammarians and descriptive linguists and lexicographers, Safire more often than not played both sides against the middle.

Note: The On Language articles in The New York Times were continued for a while by maven Ben Zimmer, who is also a regular contributor to Language Log.

Solution 2:

Professional linguist would be an accurate term for such a person, though it's not an all-encompassing one (because there are many 'grammar gurus' who do not make a living from their language expertise). We also have the terms English teacher and English professor for people who are (in principle) expert in English and disseminate their knowledge to others.

Terms like grammar Nazi and grammar police exist in response to the fact that:

1) not everyone welcomes having their linguistic practices commented on or 'corrected';

2) many self-styled experts cannot tell the difference between their own personal preferences and usages they are merely unfamiliar with, but apply their own prejudices regardless; and

3) some of them have a habit of claiming greater knowledge or expertise than they actually possess.

Incidentally, it is unlikely that I would describe myself as a grammar queen even if I possessed the necessary level of expertise to qualify for the title.

Regarding the term grammar guru, I have the following observations:

1) 'Grammar guru' does not occur in the OED as an established term.
2) The citations contained in the OED's entry for guru (of which a clone can be found here) refer solely to the term's religious usage until 1957, after which it occurs in a secular context, sometimes as a less-than-complimentary term.

(For instance, an article in the London Times dated 4 October 1968 reports the British Prime Minister Harold Wilson talking about Enoch Powell, the Conservative MP for Wolverhampton:

"In what was obviously intended to be a sneering reference, the Prime Minister in his facile glibness..described Mr. Enoch Powell as ‘the guru of Wolverhampton’."

Here, Wilson was using 'guru' as a snarky epithet for a political opponent whom he regarded as a false prophet and troublemaker — in April 1968, Powell had delivered his notorious 'Rivers of Blood' speech opposing new legislation that would make it easier for immigrants from the Commonwealth to settle in Britain.)

My attempts to pin down the earliest attested occurrence of grammar guru have been largely fruitless.

The term is not found either in Brigham Young University's Corpus of Contemporary American English (COCA), its Corpus of Historical American English (COHA), its Strathy Corpus (of Canadian English), or its British National Corpus (BNC); nor in Oxford University's BNC. I have searched other corpora too, but with the exception of BYU's Corpus of Global Web-Based English (GloWbE), which contained only a handful of what appeared to be relatively recent hits, these too were devoid of the term.

For what it's worth, the domain name grammarguru.com was (according to Whois.net) first registered in 1999.

The nearest I could come is a 1997 reference to slang guru by Jonathon Green, the renowned lexicographer of English slang, describing Eric Partridge (the author of inter alia A Dictionary of Slang and Unconventional English).