I will admit some of the best shows that I have seen can't really be watched more than once,
Also most of the crappiest.

As far as character designs go . . .
If I can tell the difference between characters without having to refer to their hair/eye colors this is a plus.
I have seen a number of shows that had that type of problem.
Generally I like to have a small amount of reality . . .
Real people usually have discernable faces and features, I think a good character designer can do that with their characters.
Even though there is a distinctive style, a good design can still have discernable characters.
Granted, you could just animate the same character numerous times, with a slightly different hair style, or something, to differentiate between them and depend on the writers or the Seiyuu fill out the character, but I like to be lazy and know just by looking.
There are other ideas going on as well....
How to get a female character to be cute/sexy or "moe"...
There are just so many things make a character attractive or repugnant.

Production designs are a little trickier....
Sometimes, "more is less", and vice versa...
One of the best animated shows I ever saw was the old, 1964-65 series,
"Jonny Quest"
The backgrounds were pretty much static and little more than good matte paintings.
The characters were animated just enough in the mecha/planes/cars to make the action seem plausible. They'd even flip the same scene several times to give the illusion of it's changing.
These didn't require 30-frame animation, the bits that were critical were animated just enough . . . Some of the best shows around today can do this...
By the same method employed badly you can get results like the episode of Gurren Lagann (episode 4) that pretty much everyone hated because the animation was kind of bad,
(Yes I'm being nice about it).
Disclaimer!
It has been said that the quality issues were intentional, in order to parody the often occurring problem of questionable quality that plagues many an anime.
ie: the cheap episodes.
(Knowing this kind of stuff is also a plus.)

A plot as well as a theme are nice things to have...
There have been instances of both being totally missing from a show, more often the plot...
Themes are pretty easy, just pick an idea and there you go... it's hard not to have one.

Plots . . . that can be a dicey proposition... it may not really show up until the end . . .
. . . and you still might not really have one when you get there.

None of those things can ensure that there is a good show, regardless of how good the other elements are.

Sometimes all it takes is a critical moment of discontinuity or just a crappy ending to screw up an otherwise good show.

All anime either has a cliché or "hook" or is made from a cliché,
the best ones don't make much out of them or can make a good story from them.
But I am old and I have seen a lot of shows,
and I grew up in the 1960's and 1970's plus I used to read A LOT,
so I might be a bit jaded.

It's still just my opinion...