Headphones are based on a full range driver that has to cover the whole audio frequency band. But developing and making a driver that can reproduce audio from say 20Hz to 20Khz with the minimum amount of gain loss at the extremes of the frequency spectrum isn't easy or cheap. So on cheaper drivers you can expect a gain loss in the bass and treble. Designers would select a suitable driver response for whatever market position the headphone is aimed at. So a more bass heavy driver would be used in say a bass heavy headphone.
In the case of cans such as the HD800 the driver is better engineered. So it has a better frequency response in the upper frequency range compared to your common variety ( read: cheap) set of cans. But few would have heard cans with less treble loss. This generates the impression that the HD800 is brighter or that the treble response is somehow incorrect. But far from it. If you own a high-end set of speakers and compare the high frequency detail from the speaker against the HD800 you'll find very little difference in the additional information that you hear. Compare that to a cheaper set of cans and soon you'll get the hang of the HD800.
The HD800 really is for people with excellent speakers. They are more likely to appreciate the similarities in terms of what can be heard.
I think this sounds quite reasonable.
I have JBL 4310s at home and one really does appreciate the clarity and sound staging the HD800s are able to provide.
It has been said that the HD800 is picky about what it is being fed by and the recording of the music.
I should note; if the HD080s are not being fed adequate power, they will take a long time to break in (least that is how it was for me. I don't know if my setup is enough for the 800 (but they sound frigging amazing and I am happy my setup works) but my HD800s recently finally "broke in" and WOW. After a year of owning them this whole extra layer just came in and swept the sound field.