These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Career Resurrection: Ultimately what hosting Late Night (and later The Tonight Show) did for him. Up to that point, he'd had some success on SNL and at least one film that did moderately well critically (Fever Pitch), but that was followed shortly by the dud that was Taxi (with Queen Latifah), and there were quite a few who thought he'd fade into obscurity. The criticism for his Corpsing on SNL didn't help his reputation, either. However, after developing some recurring bits over his first few months on the show (and hosting the 2010 Emmys to great success), his reputation was saved.
Crowning Music of Awesome: Any time he does his Neil Young impersonation. He even got the real David Crosby and Graham Nash to perform with him! And then there's his rendition of "Whip My Hair" with Bruce Springsteen...
Hilarious in Hindsight: The Family Guy episode "Don't Make Me Over" depicted Jimmy Fallon as the host of a fictitious episode of SNL with the Griffin family (featuring a made-over Meg) as the musical guest (and Peter beating up Fallon for cracking up on-camera since only Caroll Burnett has earned the right to do that, not realizing that Fallon had sex with Meg as part of the show's cold opening). Why is this Hilarious in Hindsight? Because Jimmy Fallon actually came back to host SNL for a Christmas episode in the show's 37th season. The musical guest, however, wasn't The Griffin family or even Seth MacFarlane — though he would host the first episode of season 38. It was Michael Buble.
The 1998 (season 24) Christmas episode hosted by Alec Baldwin note It was the one with the Schweddy Balls sketch had a It's a Wonderful Life-meets-A Christmas Carol-style story that shows what the future will be like if Alec Baldwin phones in his performance on this episode. The episode predicted that in 2011, Fallon would come back to host SNL. It came true (only the date they gave in the 1998 episode was inaccurate, as December 12th, 2011 did not fall on a Saturday).
Ho Yay: With Justin Timberlake. So much that during Justinís impromptu speech at his wedding reception in front of 150 guests, Jimmy interrupted him with a joke from his seat, sparking an improv between the two of them that went on for a good five or 10 minutes and had all the guests roaring with laughter. Justin admits that if it was anyone else, he would have kicked their ass.
Just Here for Godzilla: With The Roots as his backup band people who don't like his comedy claim they only watch his show to see them.
The Scrappy: On SNL, particulary during the show's 28th and 29th seasons, when he gained notoriety for cracking up during his sketches.