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When I've drunk high-IBU beers such as Mikkeller's Hop Burn High (labelled as 300 IBU) I've been able to taste and smell the hops, but I'd be hard pressed to say that it was e.g. 6 times as bitter as a 50 IBU beer.
So I was wondering if there is something about the beer that I'm missing that would indicate that it was a 300 IBU beer? Or is this more of a marketing thing when breweries label beers with such high numbers?
The brewery really needs to indicate how they get the 300 IBU measurement. Most just work it out from the hops they add using hop bitterness calculators, rather than it being an actual measurement of the bittering compounds in the beer. For highly hopped beers, the calculated IBUs can be far off compared to reality - above around 100 IBUs it becomes more and more difficult to increase the IBUs of the beer.
The isomerized alpha acids that give the bitterness are only marginally soluble in wort (they are hydrophobic), and their solubility decreases as more is dissolved: solubility decreases with lower pH and dissolving the iso-acids lowers the pH, so solubility is self-limiting.
Thus, labels with over 100 IBUs should not be considered accurate, unless the brewer has specifically measured the IBUs in the beer, rather than guessed them from the hop regime.
International Bittering Units (IBUs) are a measure of bitterness in a beer. As such, the distinguishing factor is bitterness. :)
Since hops are the primary contributor to bitterness in beer, high IBU beers also tend to be hoppy (although that's not always the case, since hops added at certain points in the brewing process contribute mostly to bitterness, while hops added at other points contribute mostly to hop aroma and flavor).
As an aside, heavier beers (specifically, those with lots of malt) will taste less bitter than a lighter beer with the same IBU. To elaborate:
there is still the complication that IBUs do not really correlate with perceived bitterness because other aspects of beer affect the perception of bitterness. For example, two beers containing 25 IBUs will be perceived very different with respect to bitterness if one beer had an O.G. of 10 °Plato and finished at 1.5 °Plato and the other beer had an O.G. of 12.5 °Plato and finished at 2.5 °Plato. Begin varying the malt bill by adding crystal malt, for example, and changing the content of various water salts and things become even muddier. This is the reason that measures like the IBU are extremely useful when used within a population of similar beers, but not so handy when looking at very different populations. - http://byo.com/light-ale/item/2084-measuring-ibus-mr-wizard
