I don't think there is a hard and fast rule for this type of uncanny valley. When I watched The Adventures of Tintin, at first I was really bothered by the cartoon characters with realistic eyes and hands. But by the end of the film I was engaged enough in the story that I didn't notice so much anymore. I suspect that a lot of our reaction to the uncanny valley is a bias that can be broken down with repeated exposure.
|These guys' eyes still freak me out|
In my experience, the most important form details for navigating the the uncanny valley seem to be the eyes and the nose. Make the eyelids too defined, and the character will fall apart. Visible skeletal structure on the hands and feet are a good target for abstraction as well. Realistic forms on the ears and lips might be distracting, but don't seem to "break" the character in the same way as the other features.
Does this mean you can never define the forms around the nose? Not necessarily. A good rule of thumb is to ask, "Do I want people to stare at this feature?" Because people's eyes will be drawn to any unusually detailed part of a stylized character. If the part they are staring at informs them about the character, then that can be a good thing. But if the nostrils aren't particularly important, then your design might be better off without them or at least without some of the structures surrounding them.
Even if you keep the forms simple, the texture on those forms can make a character disturbing. Skin pores, tissue striations, and loose hairs can be particularly offensive (in this case intended for humor):
http://pixeloo.blogspot.com/2008/03/super-real-mario-world.html Finding a more abstract version of these textures can still give you high detail without sacrificing appeal:
Of course, some of you might look at my three examples above and think that the third version is just fine, while others might feel that the middle version is already descending into the uncanny valley. What is okay depends on your audience and what you're trying to say with the style (grotesque is sometimes good).