TITLE: Why is the sequential criterion true in metric spaces? When does it fail in general?
QUESTION [5 upvotes]: I’ve read just the basics of some introductory analysis books and sometimes they show that we can characterize things like limits, continuity, compactness, etc. in terms of sequences. 
I’ve heard that these sequential criteria hold for general metric spaces, but that in topology for example one encounters situations where sequences aren’t quite sufficient, or where it’s better to consider some other object.
My questions are: 

Is there some intuition for why the sequential criterion holds in things like Euclidean space or general metric spaces, but not in some other spaces? 
Does it simply have to do with the fact that we have a metric, and if so, why does the metric “induce” such sequential criteria (versus without a metric we may not)? 
Is the notion of distance/metric captured in some way by sequences because approaching some value is equivalent to a sequence approaching that value? 
Are there any ways by which we can determine whether a general space possesses these sequential criteria? They seem quite useful.

REPLY [3 votes]: James R. Munkres, in Topology: A First Course  section 3-7, defines two reasonable weakenings of compactness. On page 178, limit point compactness is when every infinite subset of the space has a limit point. On page 179, when every sequence has a convergent subsequence, he calls it sequential compactness. It turns out that the product of two sequentially compact spaces is again sequentially compact. However, the product of two limit point compact spaces need not be limit point compact. An example is item 112 in Counterexamples in Topology by Steen and Seebach.