>>14
>>16
I wouldn't say "extreme", but like all things of very good quality, it is in the minority.
Usually the kinds of H that get the full treatment are either based off of extremely successful or popular H-games, like the ubiquitously recommended Bible Black series, or direct OAVs, usually borne on a popular artist, like Urushihara Satoshi's Front Innocent. Other OAVs, like those of what's known as La Blue Girl and Urotsukodouji, evolve over time from a particularly successful series.
There have been no fully H TV series that I know of, though, if what I hear is correct,GONZO's GANTZ has many sex scenes that live up to the rest of the animation there.
>>4
There are many OAVs that feature "regular" sex and romantic situations, but the best examples of such available in the US are already very old, such as in A Heat for All Seasons or the Can Can Bunny games, or that older Angels in the Court OAV.
Otherwise, given that the truly "best" H-animation works are based off of popular games, it usually results that when a "tame" H-game is very popular, it's usually converted into a "Real" Anime TV series or OAV, with the sex either omitted or toned down. Take for example Tsukihime, and now Fate/STAY-NIGHT.
Both games had superlative H, but are getting the full nine yards, with Tsukihime being converted to the somewhat-disappointing Shingetsutan Tsukihime, and Fate/STAY-NIGHT getting a hopefully better treatment soon.
If you want a more extreme example, take To Heart, and Air (I think). To Heart was a tame H-game that was converted to an anime series, and subsequently became more popular than it ever was as a "clean" property.
Which leaves the other, exclusively "Hentai" works to be based on fetish, of which in Japan there is very, very, very ,very much. I mention once more, Bible Black, and its new partner, Discipline.