I had a long train ride today, so I gave your game a go and actually finished all of the free content. I might have to buy the rest for the holidays.
All in all, it's a good game with a lot going for it, but there are a few things that I thought could be improved.
Like many have already said, a brief interactive tutorial could be in order, though I don't think that players familiar with the genre need one.
Then, as Richard pointed out, I think that the dictionary is questionable. Without any clues on what the words might be, I often resorted to brute forcing them. There are a lot of proper nouns, abbreviations and obscure words. I encountered words like: ANA, ALAN, ALA, CAS, NOS, ONS, CSS, CPU, VCR, GNU, UPC, COS, YRS, AUS and MOTOROLA. I love word games and puzzles, and whenever I solve (even by brute force) a difficult word, I get a kick out of it, but when I find out that the answer was something that I don't know the meaning of even after googling it, I don't get the same rush. I just get confused.
Now, this suggestion may be a bit too late, but many similar games that I've played provide a category for every level. It also makes managing the difficulty level easier as guessing VCR becomes more reasonable if you know that the category is "video". I'm also guessing that people who've been born a couple of years after I was don't even know what a VCR is. This way providing categories like "US states" or "brands" makes guessing Motorola more reasonable.
As for the audiovisual style, I liked the relaxed and calming theme, as well as the transitions. The art was, however, quite pixelated on my tablet. I've attached a screenshot of the game just in case (make sure to watch it at 100% zoom).
I found that the game had hints quite late as I was looking through the menus and the options just to see what you've done. Now, for the current monetisation method and its at times obscure words I feel that having the hints be free is quite fair, but many games that I've played have offered hints for watching an ad (or paying with in game currency). Watching an ad to reveal one letter's locations on the level would have made my play experience a lot easier a lot sooner and I would have watched a few dozen at least.
Finally, this might not be an issue, but after I completed all of the levels, the final level simply repeated. Same with other previous levels that I tried to replay afterwards. Adding a tiny notification whenever the player reaches and completes the last available level would be nice.
Now, I also have a few questions. Did you create all of those levels manually or did you generate them? Do you track player progression so that you know how many players you have at every step and if they are active so that you can plan future updates accordingly?