I have horrendous face and name blindness. I literally just got in from seeing my sister and her 10 month old and I can't picture or describe them. I have various coping mechanisms that I never saw as such- like when my sister had her baby, I wrote his name down and said it to myself a lot. I do that before I go to see her. Its like it'll be on the tip of my tongue but I don't know it! I have to double check every name I use, writing a christmas card to a sister I'll look up the name and have it in front of me. If a family member changed their looks and I saw them out of context I wouldn't recognise them, even my own mother. It has properly affected me in the past, had a chat with a mum in a playgroup and didn't say hi in the street because my mind didn't connect woman-in-playgroup with woman-in-street, even on the same day!
I also really struggle to 'picture' things, places. I can't picture people at all. If I were to try to imagine my mum, in my mind is a vague image of her living room as that's where I see her most, with 'person' in the image. I know that person is my mum, as that's what I'm going for, the emotions I feel are there, but it's vague. I have to really work to visualise, like with the counting sheep thing it's a vague blob but I know it's sheep, I know it must be wooly and have legs but that isn't the picture? Its weird!
Dreaming is also weird, I have this thing where I realise I'm dreaming but I really struggle to control anything. Again it's vague, like I know the last dream I remember was on a particular street I grew up on not because I see it and recognise, but because I 'know' it's that street. I do tend to know I'm dreaming though which is probably a whole other thread!
I love reading but I don't visualise characters or places, or create anything new. In guards guards, when I read it vimes is just 'man in uniform', the dragon is big thing, and in my head it's all set in truro town square 
not because I picture it as such, but when I first read it I was there and that's where it will forever be. Also, and this is ever weirder, I have a flat in my mind. Again vague but when I read I tend to 'set' it in this flat I created years ago. It makes reading easier, to have a place where stuff happens. I couldn't describe it well, there's a door and a corridor and bedrooms here and here, nothing sharper than that, but it's where I can comfortably place happenings.
I suspect I am very weird, now I've delved into my mind!