I wish the Scramble button in the 7-by-many only scrambled the letters I've typed instead of replacing them with the seven letters available. Though I expect it would be much more difficult to scramble 20 — or more — letters which have been entered.
I haven't spent a lot of time on the 7-by-many, but it is a different sort of challenge as others have commented. When playing on my mobile, I still find myself tapping the last letter twice to post the word and then realising — doh — that doesn't work!