I think sting is a good way to describe it.
When your tolerance is low, as it is first thing in the morning, the sting is more noticeable even with low levels of nicotine. Throughout the day as tolerance raises you need more to create the same effect. Allowing gaps between doses increases effect because you aren't constantly keeping tolerance high.
It's difficult to isolate the taste of nicotine in eliquid because of additives like pepper that are meant to create harshness.