I heard an interview with Steven the other day, and he was saying that it was just time for a life change. If you consider that he's been playing in thisband for 20 years, that's a long time to keep one job. Now, while some bands (U2 for instance) would be happy to keep it going, I think that Steven didlegitimately want to have a change in career path. Apparently he has been working on some local theatre productions and he wants to spend more time doing that,instead of touring.
In any case, it won't be the same without him. But, i've always loved Ed's lyrics/singing too.