As has been said already, it's called a "Spectrum" for a reason. We all have varying symptoms at varying degrees of intensity. Also for the case of Aspies, we're usually very intelligent, we just lack the ability to express ourselves well so people tend to think we're not. This is why I get frustrated when people say things like "oh you wouldn't know, you seem to speak ok to me"... All I can think to that is "well thank you for assuming I spent the last 27 years with my eyes shut, not learning a thing".

Some people have this misconception that we can't understand social queues, full stop. That's not always the case, after all, by the time we're adults we've had a very long time to learn from all of our social failures, to learn the unique facial queues of our family and close friends. I'm usually pretty good at picking up on non verbal queues such as facial expressions of my friends or family. Put me in a room with a stranger though and I wouldn't be so sure. I may eventually realise, but usually hours later when I'm "replaying" the conversation over in my head and I notice the things that my brain couldn't pick up on at the time of the conversation because it was too busy trying to listen to what the person was saying.

That said, sometimes even with close family and friends I can have a delayed response to facial queues. A great example of this happened yesterday; my mum told me she had lost 6lb, to which I replied "God that'll be because you've been sick this week", which I figured was the 'reassurance' she was looking for because she had in fact been rather unwell.
After I said it, I noticed her facial expression change, which then made my brain reprocess what she had said at first, that was when I realised she had her lips curved up slightly and her eyes were wider apart, this is an expression of happiness in my mother. She had in fact been mentioning it out of joy that she had lost weight without trying and I then pooped on her parade because my logical brain responded before processing all of the information.

As for empathy, this is another common misconception that people on the spectrum can't empathise. In fact, recent studies have shown that Aspies in particular may be hypersensitive to the emotions of others in part due to our common alexithymia. With the inability to describe or express our own emotions, we have a tendency to 'take on' the emotions of those around us. For example, I've always had problems where if my friend or a family member is depressed, I end up feeling this hollow nagging feeling inside which I associate with sadness. It can sometimes last for days or weeks. Now that I understand it all better, there are times where I intentionally distance myself from people if I know they are depressed.

That may sound a little harsh, but to me it's the only defence I have to keep my own emotions stable, because I don't experience emotions in a technicolour fashion like NTs, I usually get one emotion at a time which when it comes to sadness is not an enjoyable experience.

I hope this post is of help, but I'm not overly sure if it will be or not. I feel I may have rambled.