To be fair, I didn't watch every episode, but I watched a lot of them...in fact I got my 6 year old daughter hooked on the show to the point that she is angry that it is not on ALL the time like it was. So here's what I learned:
1. It's Still Getting It Done
I don't remember when I stopped watching the show on a regular basis. One of my local stations used to play the syndicated repeats along with "Family Guy" back to back when I first got married and I watched every day, but when they took it off, I stopped watching. But watching the marathon reminded me how funny a show it was and that even in later seasons as the quality diminished some, there was at least one laugh out loud moment in each episode. Whether it was a very smart allusion to a previous episode or some throw away gag that made me laugh (almost always involving Homer), it's still laugh out loud funny. I love Family Guy, but it's mostly background noise to me at this point and isn't nearly as funny.
2. It's Not Necessarily About The Actual Episodes
We were a Simpsons house from the outset. My dad loved the show, my friends loved the show and we talked about the show in school and at work. It was appointment TV at a time when that mattered. The funny thing about the marathon was the sense of nostalgia it stirred in me. I found myself remembering where I was when I saw a particular episode or what I was doing. "Bart Gets An Elephant" reminds me of St. Patrick's Day 1995. "$pringfield" reminds me of going to the casino for the first time. "Cape Feare" was my favorite single episode of a TV show for a long time. For a few days, I was thinking a lot of my early 20s and it was good and bad.
3. It Was A Dark Show
It's hard not to compare "The Simpsons" to "Family Guy" and the two shows took different paths to get where they are going tonally. At the beginning, "The Simpsons" was a dark, dark show. Even Marge was a dark character capable of some real nastiness. As seasons went on, it became more light hearted, though it wasn't afraid to get dark.
4. Speaking of Family Guy
I marvel at HOW much "Family Guy" just blatantly stole from "The Simpsons" early on. I've seen just about every episode of FG and about two thirds of "The Simpsons" and a lot of early FG episodes look like they just did a Replace All for each of the characters and slapped "Family Guy" on the cover. To be fair, I know that both shows are pastiches of family sitcoms in general, but the jokes are almost word for word in some cases.
5. Treehouse of Meh
I remember the Treehouse episodes being hit or miss with me when they were first on and I discovered that my opinion hadn't changed.
6. Flanders and Homer Are Best Friends
I like Flanders as a character and I think he's maybe the most important character in the show not named Simpson. I know that TV Tropes has an entire entry on his Flanderization as the seasons went on, but aside from the Simpson family, has there been a more dynamic and rounded character on the show (or even TV I'd argue). Flanders wouldn't be Flanders if it weren't for Homer and vice versa. Their relationship is one of the most complex on all of television and it has been fleshed out on so many levels that it's not unlike most friendships in the real world.
7. World Building and Canon Wobbliness
You say world building and you think instantly of epic fantasy, but world building is just important in any genre that you write. I discovered that this summer when I was writing my YA contemporary book. These characters occupy my version of our world and I need to build that world accordingly (I started a series of world building posts that I'll get back to). Like all animated shows, "The Simpsons" has created a vivid and vibrant world for their characters to occupy. Springfield is as significant and important as a world as Middle Earth, Westeros or the Four Nations of Avatar with as much development and backstory as those worlds. The thing a show like The Simpsons has to do (25 seasons) is be flexible with their world and "canon" when they need to be. Every episode is almost a retcon in and of it self...and that works. That being said, there are still little bits that thread the show over the course of 25 seasons.
8. I'm A Sideshowmaniac.
Through some weird cosmic convergence I managed to see every Sideshow Bob episode in the series. I did this without intent (I did DVR "Cape Feare" because it's one of my favorite episodes of all time) or planning. My daughter (who watched almost every episode with me) recognizes Sideshow Bob on sight now. He's my favorite character on the show. There may not be a more complex villain in the history of TV. Seriously. He rivals Zuko from Avatar. And he changes (an important motif of the show actually) from year to year.
9. It's About Characters
Obviously the members of the Simpson family are the focus, but "The Simpsons" rivals my beloved A Song of Ice and Fire for named characters...and not just named characters, but characters that are fully developed and even more real than some characters on most sitcoms today...I'm looking at you "Modern Family." From obvious characters like Moe and Barney to Duffman and Moleman, these are well rounded characters that even in minor moments get to shine. As a writer of big, sweeping epic fantasies, there's a lot to learn from what they do. I've talked about Ned Flanders earlier, but characters like Nelson, who evolved from a simple bully into a template to writing the complex bully that led to characters like Buford (Phineas & Ferb) and David Karofsky (Glee), Millhouse, the toede and lickspittle that was given ample opportunity to shine when the light was cast upon him, the aforementioned Moe, perhaps the most tragic character and maybe the show's moral compass, the codependent Principal Skinner, the acerbic Edna Krabapple and the underrated, apathetic Ms. Hoover just pop into my mind. As a writer, there's plenty to study while watching "The Simpsons."
10. Complex Character Relationships
Man, there were a ton and too many to list. From the familial relationship of the Simpsons to the dynamic friendships and interactions between the Simpson family to the external relationships between the denizens of Springfield. The first time through I never noticed how integral a part of the show they were, but as I watched as a writer, especially one that writes a lot about relationship dynamics, I noticed they were a major motif the writers played with. There were a few that struck me. Nelson (who became one of my favorite characters on the show) and Lisa's remarkably complex relationship is fascinating and well fleshed out on several levels and maintained consistently. Homer's relationship with Monty Burns always stands out. There are countless others that Burns could interact with, but their constant interplay often drives the plot of episodes. Lisa's solitude and isolation is done in varying degrees of success but it's never better than when she makes friends for an episode only to have them disappear. It's a nice touch that this mirror's Marge's own isolation and solitude and done in the same manner. And there's an entire essay to be written about Carl and Lenny.
So, there it is. I'm sure that the Internet has done the whole Every Simpsons Ever thing to death and you know me, I'm a sheep, I follow the crowd.