It is probably fair enough to say that letter quality truly resides in the eye of the beholder, and as such, each opinion is valuable.
This said, in declaring what letters are of better or lesser quality, several important factors should be considered, and even then there is not universal agreement. Some of these factors include:
Letter commonality within acronymns
Language
End User potential
Memorability/Branding
Common Usage
On 3character.com, I think it is fair to state that by general concensus the premium letters are: A, B, C, D, E, F, G, H, I, L, M, N, O, P, R, S, T. Not every single person agrees on this, but very few would argue that these letters are not of premium quality given their commonality in many major languages, commonality in business acronym usage and their increased commonality in end-user sales among LLL domains.
It can get a little grayer on the lesser high quality and lower quality letters. For example, within the english language, Q, X, Y, and Z are much less frequently found and used as the first letter of many words or names. RJ pointed out a very helpful site in his post that shows the commonality of letter usage in the english language. This said, commonality of usage and commonality of use as the first letter of a word or name are different concerns to consider. E is by far the most used letter in composing english words, and C and T are used somewhat less in word composition, but yet are very common as the first letter of a name or word. Likewise, X is very rarely used in the composition of english words, but even more rarely is it used as the first letter of an english word or name (and acronym domains are based on words and names).
So far our focus has been on letter quality in the english language, but as we all recognize, acronym domains are valuable in many other languages as well, and this can place a cloud on what otherwise seems a simple science. For example, Z is not a widely used first letter for words or names in the english language, but is very common in the german language. Someone from Germany might have a very different opinion than someone from the US as to how valuable Z might be as a letter in an acronym or LLL domain.
In the consideration of premium, lesser high quality and lower quality letters, the only real way to guage their placement in these categories is to seek general concensus understandings, which should consider end-user sales by letter type and end-user desire and use for each letter. Though opinions vary on this, it is most generally found that the letters fall in these categories:
A, B, C, D, E, F, G, H, I, L, M, N, O, P, R, S, T are considered premium letters.
Lesser high quality letters include: J, K, U, V, W.
Lower quality letters include: Q, X, Y, Z.
It's fair to differ with this general concensus, but it is also helpful to try to place these letters in fair general categories based on the factors above, as well as to consider one of the most important factors of all...end-user sales data (one of the greatest strengths of LLL domains for wholesale valuation purposes).
If you or I have a difference of opinion with these categorizations, it's really alright that we can all agree to disagree. End user sales and end-user usage really win the day and by far trump our own opinions.
So here we are again...perhaps it is all in the eye of the beholder...and most would agree that so far even the beholder of the worst LLL.com combination can't complain too much.
Thanks for a great thread...I'm very much appreciative of the various thoughts.

zesty