Did you purge the contents sitting in the tap before tasting? If not basically what you put in the fermenter first is what your tasting. Generally bitterness will mellow in time, bottles anyway but keg use early I'm not sure.
Cheers
Pleased to here pipsyboy. As your stout gets older and matures in the bottle it gets much better. Try and "hide" say six bottles and drink one every 6 months to get an idea how its improving and make some notes. I'm currently drinking a milk stout just over 3 years in the bottle, absolutely delicious. (how do I keep them that long? brew heaps of stouts during winter and store some away).
Cheers