Well, I said "a little more precise" and "I know that none of those terms are exact", so yeah.
At least with sufficient\insufficient you understand if the rating is positive or negative, whilst with "average" it could mean anything.
(we were talking about the term, not about tastes)

Just to make sure we're on the same page as to what we each mean, I'll pretend I have to codify these terms for a review score system. I would define the terms we're discussing as follows:
- Average = OK within its genre, but it's got problems or shortcomings that keep it from being considered even good, let along excellent
- Sufficient = Average or above (for the person doing the evaluating)
- Insufficient = Below average (for the person doing the evaluating)
That's why I say "sufficient" and "insufficient" are still very relative terms -- looked at in light of the above definitions, If I had to assign a rating (or range of possible ratings) on the 1-5 scale to the above terms, I would say "average" = 3, "insufficient" would equal 1-2, and "sufficient" would mean 3-5.

I can understand your reasoning, but.. you're considering something below "sufficient" as "average" and "OK", but not yet "insufficient"? Mm..
It seems like you're actually looking for a neutral value, but using wrong terms.
I would maybe agree with that choice, if only 5\10 wouldn't be considered a fail result almost everywhere.

(3\5 somehow feels more positive.. maybe also because it translates to 6\10)

I think I just explained it rather confusingly. :) I didn't mean that those three terms together composed a hypothetical rating scale -- I was just defining what I think "average" should mean on such a scale, and then defining your two words in terms of how they relate to MY word. Does that make better sense?
And that's the problem -- ratings for game reviews have been skewed horribly by the games news industry over the years. Like I said in an earlier post, it's pointless to have 1-6 all mean "don't play it", have 7 be the "mediocre" rating, and only have three different levels of "good". (I actually like the simple 5-point scale that Falkenherz quoted -- it leaves less room for equivocation.)
Technically, translating 3/5 to a 10-point scale would make it 5.5/10, at least if you're going for a single usable rating that's dead center on the scale (...then again, a 10-point scale that allows half-point increments isn't really a 10-point scale). Otherwise, it'd be equivalent to a range: "5 to 6 out of 10". ;D

(Sorry if I'm going a bit too deep with this digression. It's kinda interesting, though, and I'm bored! =D )

But 5=insufficient isn't a "problem" of game reviews. You get that right from school!
I agree, instead, that 7=mediocre is absurd.
I don't like the 5-point scale because it's too limited for ranking many titles.

Well, for that you just need the scale 0-10 instead of 1-10.
About the half-point, it's true: it becomes a 20-point scale.
However, I prefer it as it's more accurate (even if I use it *10).

Lol, no problem.
I just wish that scores could be more standard XD

If a few people in one random GOG forum thread can't agree on what makes sense, I don't think there's a very good chance of any one system becoming widely accepted by all reviewers and the general public. :D

Well I know, that's why it was just a wish :P