Here are my qualifications for the ratings:

*****
Masterpiece. Almost every song has to be strong, memorable, stand-out track. Listening experience is so fluent that you don't even think about skipping any tracks.

****½
Great piece of work. The release consists of many highlights but in comparison to full five star release, it may be a bit more uneven on some spots, i.e. it might include too many regular, standard, generic songs. Nonetheless, the release is highly consistent as a whole.

****
Very good, meaning roughly a half of its material is worthy to be called highlight. There are many albums that fail a little bit on this department, e.g. the record has 4 highlight tracks out of 10, so I can't give the four star rating even the whole album otherwise would be solid. Four stars is always the most difficult rating to decide, whether the release is worthy or not. A watershed basically.

***½
Good, meaning the release has to include 3 highlight songs and the whole listening experience has to be quite solid, at least. Usually, it needs a good opener and a good closure. The stuff between can be a little uneven sometimes. If I'm too bored at the end of the listening experience or it's just too laborious to get through as a whole, even it had a a couple of great individual tracks, the release is only worthy of three stars. If the album/EP has less than eight tracks, fewer highlight tracks may suffice.

***
OK, decent...not too bad, not too good. Requirement for this rating is at least one highlight track and decent, not too boring listening experience from beginning to end.

**½
Average, one great track short of OK. Not too shabby but you get frustrated in the end, it shows some potential but it's missing something. It's passable background music that doesn't irritate you too much.

**
Still listenable, but there's too much mediocre stuff in between. It may have a couple of good tracks, but you rarely give them a listen due to the overall blandness.

*½
Quite bad. You may give it a listen after a couple years out of sheer curiosity and/or amnesia asking yourself, if it's REALLY that bad/forgettable record.

*
Just bad. I don't have this kind of annoying music in my CD collection or on my Winamp playlist. It's not the absolute worst but not too far from it.

½
Timo Tolkki's Saana album. And current pop music basically, particularly modern rap / hip hop and EDM artists on the radio. (As a side note, Rap specifically is a genre I have a hard time to digest, and I'm least interested to discover it. But I don't judge people who enjoy it. Each to his/her own.)

P.S. Live recordings and compilation albums are a different thing, however. In their cases, the overall feel is all that counts. Does the compilation album include all the right songs? Does it have anything extra to provide, which makes it a worthwhile purchase even for a huge fan? Did I become a fan after listening to this collection? How does the singer sound on a live album, can he/she make the song better in a live environment? How does the band sound live, is there any crowd participation? Does the live album sound too fake, can it fool me to believe all that I'm hearing could be recorded live? After all, I love plenty of live albums that apparently are quite fake in fact.

In any case, four stars is yet again the standard that separates the wheat from the chaff.