I did critical thinking at AS, did it as a 5th AS and somehow managed to get an A! I didn't find it particularly a strain managing it at all, although you need to find out how it will be taught as we did it on 2 50 min lessons a week. I wouldn't say there is anything specific that makes one person better at it than another, just an open mind i think.
It is quite challenging, but interesting at the same time- its a subject of learning skills, and practise really does pay off. You get better as you go along, and it comes in useful in other subjects and other things in general! Contentwise, you learn about flaws in arguments, their names and how to spot them, how to identify structures of arguments- a favourute question is identify the main conclusion in the passage! You also learn about how to identify how credible a piece of evidence is.
Hope this helps and anything else you want to know, ask away xx