I liked Muppets Tonight and do miss it a little bit, but it did have its issues. One in particular was the heavily featured new cast additions taking over classic character roles instead of being phased in.
Clifford: Host (Kermit's former role)
Rizzo: Production assistant (Scooter's former role)
Gonzo: Veteran Muppet (He almost seems to take more of a Fozzie "second banana" position)
Seymour & Pepe: Staff (The best most Muppety additions. They were one of the two successful comedic duos on the show.)
Johnny & Sal: a self-deluded almost Link-type persona, but as a humanoid crooner paired with his monkey assistant. (The other successful comedic duo on the show.)
Bobo: Security guard (A funny backstage character in the classic tradition.)
Andy and Randy Pig: Crew (Overstayed their welcome.)
Kermit the Frog: Producer (He's aloof and uninvolved most of the time - very different from the Muppet Show or the Henson Hour. The show's greatest mistake.)
Spamela: Resident Diva Pig (Upstaged Piggy in a way Annie Sue never did. Funny, but overexposed and made fans miss the real deal.)
Nigel: Director (Funny, but given little opportunity to shine. Eugene was his assistant.)
Fozzie, Miss Piggy, Animal, Sam, Bunsen, Beaker and other classic cast: guest appearances (but seemed to be more heavily used toward the end.)
Van Neuter & Mulch: Resident scientists (Mulch is a classic character and the duo was funny at times, but they eclipsed the more familiar Bunsen & Beaker duo.)
Jowls, Zippity Zap and the Muppets Tonight Band: Replaced the Electric Mayhem (Very understated characters and a poor decision all around.)
David Hogselhof, the Swinetrek cast, Mr Poodlepants, Bill the Bubble Guy and others: Took over for Link, Strangepork, Marvin Suggs, Lew Zeland and other, better classic bit-part characters.
Ernst Stavros Grouper: Station owner (the JP Grosse roll, but JP was strangely a lowly crew member.)
Beauregard, George the Janitor, Janice, Rowlf and other classic characters: popped up inthe background from time to time.
I'm forgetting several characters here, but that's the gist. Again, a good show, but Kermit should always play host and the Electric Mayhem should always be the house band IMHO.