Obviously, I know they don't all follow the same rules, but the rules they both follow, the follow equally. Yes, the lead section should clarify what a list includes, but a lead section of a regular article should also clarify what is included in the rest of that. I didn't mean that lists and articles are exactly the same and should be treated the same, but with regards to the Lead, it must do the same thing.
For a list of US presidents, I believe you have to say what the US president is, not necessarily his constitutional duties, but what it means to be President, what the position is, to provide context for the reader. I would also say how many there have been, who was the first, and who is the current. I may also include other information, such as how long a person is President for (a term of four years) and then maybe say who has held consecutive terms, who has held office more than once (separately), who has died while in office. An overview of the presidents, not what the president does. That's just off the top of my very head, at 2.30am, mind you.
No, plot summaries should not be included in a list of episodes (unless there are no season articles). FYI, I have written 8 episodic Featured lists (List of Degrassi: The Next Generation episodes and each season of that series) and 3 other Featured lists (Chief Mouser to the Cabinet Office, List of Scripps National Spelling Bee champions and List of Blue Peter presenters, as well as two Featured articles related to TV series (Degrassi: The Next Generation and Mother and Child Reunion), as well as the Leads for the FLC List of 7th Heaven episodes and former FL List of The Sopranos episodes. 
While I may not be expressing myself as well as I might at 10am, I do know the different between a list and an article, and am not suggesting that any old information is stuck in there to bulk it out, but also, a little more than simply "Avatar: The Last Airbender is a 61-episode Emmy award-winning American animated television series written and created by Michael Dante DiMartino and Bryan Konietzko. It aired from February, 2005 to July, 2008." I would say how many seasons, whether the episodes have been released on DVD, or any other new media outlets such as iTunes or something. I wouldn't include the current second paragraph, though.  (talk • contribs • email)