Thoughts (spoilers)
It was a mixed bag for me, but overall it was very good. Obviously Jordan is a great director (Company of Wolves is in my all time top ten), and the cinematography, score, acting, production design is impeccable. Somehow it didn't wholly work for me. It's a second tier vampire movie, not a first tier one like other 21st c. movies like Jay Reel's Dawn, The Moth Diaries, and Priest.
Because it switched periods it seemed to shift from a pre-Dracula vampire tale to a modern arthouse one like Let The Right One In.
Some tropes, themes, and symbols it shares with vampire, urban fantasy, paranormal romance, and period movies and other fiction:
vamp battle of the sexes - We Are The Night (reversed)
daywalkers - common to pre-20th c. vampire literature
vamps & seashores - Jean Rollin
Isle of the Dead - Isle of the Dead (Val Lewton), Piano Tuner of Earthquakes, Cemetery Man, others; One big difference in Byzantium is that its island does not look like Bocklin's painting and there is the direct transformative property
lonely boy/girl is vamp's friend - Let The Right One In
climax: a 'good' character looks like she/he is going to kill another 'good' character but kills bad guy instead - very, very common trope across genres; The Believers
restrained vampire/witch is annoyed with less restrained relative - common; The Silver Kiss (novel), Kiss of the Damned, Vampire Diaries, Beautiful Creatures
vampire/witch police out to punish protagonist - very common in urban fantasy and paranormal romance
adolescent vamp goes to class - Vampire Diaries (1991 and TV series), Twilight (not so Let The Right One In, Silver Kiss)
older vamp has Eastern European accent - very common in urban fantasy
leans in seemingly to bite, but it turns out to be just an affectionate gesture - Twilight (get over it, Twi-haters)
I have a secret but I'll tell you anyway: I'm a vampire/immortal! - many paranormal romances, Tuck Everlasting
terminally ill person is turned - Vamps
sex industry vamp - Vampire Boy comic, Bordello of Blood, Vamp, Dusk Till Dawn
Carmilla, Camilla etc. - Carmilla (novella) and adaptations
vamp as angel of death - Dawn by Jay Reel (a great no-budget movie that few have seen), To Sleep With a Vampire (suicidal instead of terminally ill)
I liked how in the action scenes there was no wire-fu, slo-mo, ramping, super-strength etc., a refreshing change from the usual urban fantasy.
The island cave process of becoming a vampire was novel, but it did turn into 'Everyone into the cave' by the end - by which time we had seen four people do some step of the procedure. I'm sure it wasn't intended, but it reminded me of Luke's encounter with himself on Dagobah in Empire Strikes Back and its South Park parody.
It's been done before, but the theme of misogyny and patriarchy is well done, as was the 19th c period atmosphere.
Why didn't it totally work for me? I'm not sure. Darvell was a key character, but he seemed underdeveloped except for "I'm a product of a patriarchal world but I wish we didn't have to be such dicks." (Maybe that's all we needed to know about him.) I bet the Brotherhood put him up to recruiting Ruthven (name obviously referring to Polidori's story). The dramatic lynchpin is the believability of a) the mother-daughter conflict and b) the daughter's love for the new guy. I was sold on the former but only half-sold on the latter. He wasn't the first; the mother must have killed the others. I liked the unique aspects (thumbnail weapon, island) and found the urban fantasy cliches a bit tiresome. Then again, I'm more forgiving of cliches with some other movies. So maybe it's some ineffable quality of it not coming together 100% for me.
Not among the very best, but head and shoulders above most of the vamp movie herd. 8/10.
Update: Maybe my biggest issue was my very high expectations. Plus, I've watched nearly 300 vampire movies so some things that appear unique to a lot of viewers I've seen before.