I performed an experiment a year or so ago using a 4 gallons of wort split into four 1 gallon batches. All four had a single
60 min.) hop addition with four different hops. I adjusted the amount of each hop, according to its AA%, to get roughly the same level of IBU's. I split a pack of dry yeast among all four gallons and fermented them all side by side.
There is definitely a different flavor from each hop! Each beer had it's own individual character.
I noticed that some hops contribute a more "harsh" bitterness than others.
I've read somewhere that using larger amounts of low alpha hops, as opposed to smaller amounts of high alpha hops, to get to the same level of IBUs will give a "smother" bitterness. I haven't experimented with this theory yet.
I've also heard it said that "aroma" hops will also contribute somewhat to the overall bitterness of the beer.