Nineoaks, I've been trying out some 10-letter puzzles, and I don't have any strong impressions about how they compare with the 9-letter puzzles, as a playing experience.
They will tend to have more words, but this won't necessarily mean they take longer to play. I suspect most of us spend a lot of time looking for those last few words. This may not change very much.
One thing that some regular players might appreciate is seeing some new words: the 10-letter words obviously, but there will also probably be some shorter words appearing in puzzles for the first time.
Those who try the new puzzles once they start will make up their own minds about them. All the existing 9-letter options will continue of course, with 10-letter puzzles as an extra option.
TRex, are you crazy? You want to bring in a new award that will give more incentive to people to pile onto the site whenever a new puzzle starts up?
Actually, the first to get a rosette or trophy do get a reward now - they will be on top of the scoreboard for the life of the puzzle. More precisely, the first trophy winner is on the top of the board in the rare-words-inclusive view, and the first rosette winner is on top in the rare-words-excluded view. Perhaps this is one of the carrots that gets people to the site at puzzle start time.