My commisioner pays for a PLUS league free of charge to us, so thats good. As for the features, nothing realy stands out to me. The statracker is nice though, and live updating is always good. Also it provides you with the rotowire draft kit, although i find it pretty useless.
As for keeper leagues ive never done one on yahoo so i couldnt tell you for sure. I think the commisioner would have to do it on their own. I am not teams but as far ask i know there is not "keeper" option.