As Filburt says; but also note that it's usually better to write
test="not(Count = 'N/A')"
If there's exactly one Count element they mean the same thing, but if there's no Count, or if there are several, then the meanings are different.
6 YEARS LATER
Since this answer seems to have become popular, but may be a little cryptic to some readers, let me expand it.
The "=" and "!=" operator in XPath can compare two sets of values. In general, if A and B are sets of values, then "=" returns true if there is any pair of values from A and B that are equal, while "!=" returns true if there is any pair that are unequal.
In the common case where A selects zero-or-one nodes, and B is a constant (say "NA"), this means that not(A = "NA")
returns true if A is either absent, or has a value not equal to "NA". By contrast, A != "NA"
returns true if A is present and not equal to "NA". Usually you want the "absent" case to be treated as "not equal", which means that not(A = "NA")
is the appropriate formulation.