Another wordlist surprise and two UI suggestions.
My mother got a puzzle containing HAVES (as in the opposite of "have-nots," I guess!). I think that's not a word; at least it shouldn't be in the answer wordlist.
My wife once clicked "Skip" basically by accident — thinking that it would skip to the next square, or word, or something — and found that what it means is "Skip to the next puzzle," with (AFAIK) no way to get back your progress on the puzzle you just skipped. Which was disconcerting. But that mistake happens only once per player... unless they happen to fat-finger "Skip" when they meant "Hint", I guess! Maybe "Skip" should pop up a modal dialog, both explaining what it's proposing to do ("Give up and skip to the next puzzle? Yes / No") and also giving the player a fatter target for their next tap in case they tapped "Skip" by accident.
Another thing I've noticed myself having trouble with is when a group of letters contains duplicates: if the group is like "LLSS" and I want to fill them all in quickly, I find it most natural to double-tap "L" then double-tap "S", as opposed to tapping the first "L", the second "L", the first "S", the second "S". Right now, double-tapping doesn't work; it will put the first "L" in the first cell, then undo that and put the first "L" into the second cell instead. I think it should be able to figure out what I mean.
Similarly, if I've tapped the first "L" and the second "S" to fill the first two cells, and then for the third cell I tap the first "L" again, I think it should be smart enough to act as if I'd tapped the second "L". Because obviously I don't actually care which of the two "L"s fills that cell — equal letters are fungible as far as the player is concerned.
Oh, and it occurred to me to wonder: Is it possible by random chance that one of the letter-groups displayed at the bottom of the puzzle will happen to spell out something vulgar? (If you bother thinking about that issue at all, then I think you could address it by blacklisting certain letter-groups, i.e. shipping a blacklist of anagrams (or even partial anagrams) of vulgar words, instead of a list of vulgar words itself. Or if you sort each letter-group as "consonants first, then vowels," it probably never visibly forms a word.)