Any show that lasts over 20 seasons and is still continues to be entertaining and funny earns my respect. Many other shows that last for a long time such as SpongeBob or Family Guy tend to decline in quality. The episode plots become more simplistic, the character's personalities are botched up beyond recognition, and you can basically tell that the writers don't care anymore. But with this show I can watch an episode from any season and it will still feel like the same show. In fact one of the newer episodes they made about internet piracy I would consider one of this show's best. The fact that this show can last for so long and still be of quality is one of the things that makes it so good.
The reason this show lasted so long is because of how well developed it is. The episode plots are creative, They go well beyond the typical sitcom. The character development is excellent. All of the characters, even supporting characters such as Moe and Mr. Burns have personalities. Homer although is a jerk sometimes is generally a warm, likeable character. Bart is annoying, disgusting, and immature, but he is presented in a way that makes it funny. Nelson, unlike many other bully characters has a personality beyond just being a bully. There are other great characters too.
The best thing about this show however is that it manages to be both entertaining and funny without using a lot of vulgarities and sex humor. Groening's strategy of humor is to use creativity and good writing to make something funny. Unlike McFarlane's strategy which is to throw a bunch of fart and sex jokes into something and that will make it funny. I find myself laughing a lot more in an episode of the Simpsons than family guy or American dad.
With it's well developed plot, good use of characters, and witty, creative humor. The Simpsons was and always will be an animation classic.