I've built a some word games over the years (e.g., wordwhile.com, omiword.com), and one thing I've discovered is that players find it very unsatisfying when they enter a real, valid word and the game rejects it. I imagine that the timer would amplify that sense of unfairness.

Granted, there are also people who get annoyed when the game seems too accepting of unusual words, but if you can point them to a valid dictionary definition for that oddball word, they usually accept it without argument.

I'm one of those people who get annoyed if it's too loose. But I can just impose that restriction on myself and look down on people who rely on obscure words.