Series that run for that length of time always lose something when one of the original characters leaves. I did feel that he started to overplay the quirky, jokey side of O'Neill towards the latter end of his series. If he'd have stayed on I think he would have needed to tone it down a little before it got annoying.
As for the younger bit, I'm sure you would look a lot younger with $100,000 plastic surgery, makeover, screen make-up and soft focus.on your side ;-)