Well said. That is indeed the case with most adaptations, though there are a few select ones that are very good, such as the examples you mentioned. I would like to mention again though that I think the Utawarerumono anime is a good adaptation of the VN, even though the VN is still far superior. I consider it to be pretty impossible to cram EVERYTHING from a big VN into an anime though, since certain scenes/dialogue don't make the transition into anime all that well. Therefore I don't believe that everything from a VN has to be included in a potential anime version, for that anime to be a good adaptation.
Anyway, imagine how awesome it would be if they made a proper anime series out of the Heaven's Feel route from the Fate/stay night VN, in the same level of quality as that seen in the Fate/Zero anime! I seriously doubt that would ever happen, but it's still my dream! <3
It could happen! I see Fate/Zero stuff all over Akihabara. Maybe they could do a high-quality miniseries for Heaven's Feel...