I personally think the average should be around 6, but it doesn't seem like the average game scores a 6. So I ran a quick search on Metacritic and those are the results one gets:

PS4 has 847 games, number 424 is a 73.
XB1 has 519 games, number 260 is a 73.
Wii U has 184 games, number 92 is a 76.
3DS has 356 games, number 178 is a 71.
PSV has 246 games, number 123 is a 72.

Now, Metacritic doesn't count the number of pages for the older platforms, but it seems, going by the number of pages and the number of titles on each, the PS2 average is around 69, the DS around 67, the PS3 around 71, the X360 around 70 and the Wii an astonishing 63. Thus, it seems like the average score is actually creeping up, not down, unlike the belief of some who said critics this generation have become more rigorous. Only the number of games scoring 90+ seem to have fallen, but multiple collections, DLCs and sport tiles used to make it to 90+ scores in the past (more than 40% of 90+ titles used to be those).

And that's counting a higher number of reviews nowadays, so newer platforms have a higher chance of including games which, by the lack of reviews on older consoles, wouldn't have made it to metacritic due to the lack of a number of necessary reviews.

As a side note, it seems like 600 titles this generation alone have scored higher than Mass Effect: Andromeda.

Well, almost any movie or tv-show gets reviewed.

In games there is already a pre-selection of games worthy a review, there are thousands of games that don't get enough reviews to show up in MetaCritic or GameRankings.

Considering games in 8-9 range are pretty average, I'd say that's the "average" score. Besides, any AAA title there is that get less than 9 is not worth playing anyway. That's what the reviews have taught me.

In my school you pass an exam with at least 75/100, so for me if a game score below that, i assume it's bad.

Around a 6, I'd say. I've enjoyed plenty of games that were in the high 60's on metacritic. Likewise, I've found a lot of games in the 80's and 90's to not only be bland or uninteresting, but also often suffer from major flaws or technical issues that are somehow glossed over in reviews.
Reviews are just suggestions, they don't say all that much about how you'll feel about a title.

Around 7 I see as an average game. Good game, but not great, as average story and gameplay, nothing new, fresh or innovating. Kind of a safe, lighter game.

Once a game slips into the 6 range, that's when I get concerned. It could mean the game has problems which could be detrimental to the game, such as glitches, or parts of the game lacking in quality.

That's how I see it.

To me an average game is a 6.

I go by
1: unplayably bad (gamebreaking bugs; unfinished levels; assets ripped from google images etc.)
2: Severe technical and qualitative problems, but technically playable to completion.
3: Just a bad game. It's not broken, or offensively bad, but it's mechanics aren't fun, the controls aren't tight, the camera is a nightmare and the visuals and story are nothing special or bad.
4: Below Average. There might have been something good in there, but the negatives outweigh the positives by far.
5: Troubled but cute, or a game wich reaches very high highs but is hampered by very low lows.
6: Mediocre, a game that is servicable but boring in all of it's aspects with nothing special about it.
7: Good game, nothing too terribly special though.
8: Great Game, fun to play. Coherent in it's game and visual design, minimal technical problems (i.e. no, 'fighting the camera, etc.)
9: Outstanding game, better than most. Tight enjoyable gameplay, outstanding story, great visual design, good voice acting, little to no technical problems, etc.
10: A truly special expierience. It might have problems here and there but the positives so overwhelmingly outweigh any negatives that it doesn't end up mattering.

So for me the 5-6 range is reserved for average games, with the 5 having more interesting but ultimately worse games than 6.

5/10