Yeah it's pretty awful now. Kelso is gone, Foreman is gone, and this new guy that they introduced is totally obnoxious. It's the "Fez and Jackie" show now...ugh. They should have just ended it last season.
Both wanted to concentrate on their film careers. I don't know the reason they gave for Kelso leaving, but Eric left to do something like the Peace Corps in Africa.
Just as an FYI, if you didn't know... the producers are trying to get Topher Grace and Kutcher back for the season/series finale.