(Semi-spoiler alert. I'm up to page 553.) This book is not conducive to a good night's sleep. Even when I knock off at a not-completely-outrageous hour, I lie awake worrying and thinking about it.
A curious thing: Grindelwald's reaction to V. If Grindelwald was this great dark wizard, would he not have welcomed V? Wouldn't V spring him loose? And wouldn't G be bitter at Dumbledore, rather than trying to prevent V from taking the wand?
Along Ollivander's comments about how wands are fussy about whether they take to new masters, I wonder whether the Elder Wand feels it has been properly taken by V?
Does the last horcrux lie in the Chamber of Secrets? ("Secrets" is plural, after all.) Perhaps V thinks that Harry doesn't know how to open it? (After all, it was the diary-Tom-Riddle who opened it last time, right?)
A possible small flaw in the story: when Kreacher came to Bellatrix with info (in the last book), shouldn't V have been surprised to discover that Kreacher was still alive? And he ought to have been very curious about it (as in, how did you escape my lake?), though it would be like him to not pay attention to the abilities of "inferior" creatures. On the other hand, given what was at the lake, he should have been supremely alarmed that anyone alive knew his secret.