You'd basically have just lost out on several gimmick matches and certain moments in history.
Ok, the guy's been there for 20 years. Debuted in Hogan era, stayed through Monday Night Wars, adapted for Attitude Era, adapted again for what i call the 'technical' era where guys like Benoit, Angle and Lesnar were the top attractions, and is still going strong even now despite his health.
He's put countless guys over, never seemingly used his power to get his own way, and is a locker room leader with everyone's resect.
He's also seemingly been the one to stand in the way of the guys that do/did abuse their status back in the day. I've heard a couple of stories where HHH and HBK (this was back in HBK's less respectable days) were fucking someone about, and Taker was the one to step in.
For example, he was one of the first to tell Bret that Shawn and Hunter were fully involved in the Screwjob.
Another example HHH gives on the HBK DVD, is where Shawn was smacked out of his head on drugs one night that Taker came and berated him to get his life in order.
But in terms of WWE's success? I think they could have easily maintained the level of success that they have had without him being there.
Austin, Rock, and all those other guys that have been mentioned didn't become mega stars because of Taker. Mankind did maybe, but he had an awesome fued with HBK that year as well, and after his Taker fued, he went back to the mid-card, to the point where after he flew off the Cell, people still boo'ed him and disrespected him the following night that he went in the back and ranted about it to Paul Bearer.
Glen Jacobs would have probably been given an even shitter gimmick than his dentist one, but they obviously felt Glen was an asset worth keeping, otherwise he wouldn't be there 13 years later, and he wouldn't be having a 'JBL run' with the title now. So no, there'd be no Kane, but Glen Jacobs would probably have stuck around in the 'E' for a while longer regardless.
I'll have to come back to this, have more to say, but lunch break's over.