なかなか means mildly, moderately. The origin is
中中, 'middle-middle'. It is lower in degree than
Below is slightly technical.
Elaborating on rintaun's point,
なかなか has another usage: used with a negative expression. There are words that can be only used within negative environments (and some other environments, subsumed under what are technically called downward entailment environments).
決して 食べない [Japanese example]
全然 食べない [Japanese example]
おかず しか 食べない [Japanese example]
He does not study at all [English example]
I am not paying a red cent [English example]
Sometimes, a word can be used either under this usage or under a different usage:
He cannot eat any-thing. [Negative usage]
He can eat any-thing. [Different usage]
Depending on the usage, the scale differs.
なかなか is an instance of such word. It has a usage that requires a negative environment.
彼は なかなか 食べない [Negative usage]
彼は なかなか よく食べる [Different usage]
In this negative usage, the degree of
なかなか is stronger.