This is the first full season in HD. This is also the first season in a long time where there were several really good episodes. The season average is its highest since Season 15. Treehouse of Horror XX was the first episode I rated at least an 8 since Season 18's The Boys of Bummer and there haven't been this many 8.0+ episodes (three) since Season 14 (which had five such episodes). Unfortunately, the season still possessed many clunkers as well, most notably a stretch of three out of four such episodes where comedy took a complete back seat to providing some message of some sort. Its hard to tell if the few great episodes this season contained is a fluke, or a sign of better things to come.
Also, some things I've noticed since the switch to HD but really noticeable this season: couch gags often have their own music rather than the opening theme (or, in one awful instance, the entire opening replaced with a pop song), episodes on several occasions ending with a slideshow of slightly relevant pictures, and Lisa being treated particularly well by writers as of late.
Season 21 Overall Score: 6.77
Favorite Season 21 Episode: Treehouse of Horror XX
Least Favorite Season 21 Episode: The Greatest Story Ever D'ohed
01) Homer the Whopper
The comic book guy publishes a successful comic book series about Everyman, a guy who absorbs comic book powers, and he casts Homer as the hero in the eventual movie.
02) Bart Gets a Z
In an attempt to get Mrs. Krabappel to be more fun, Bart spikes her coffee. Edna's resulting behavior gets her fired, and while her replacement isn't that bad Bart still feels guilty about what he did.
03) The Great Wife Hope
Marge learns about the ever-violent sport of mixed martial arts and ends up in a literal fight to have one of its leagues shut down.
04) Treehouse of Horror XX
Dial M for Murder: Bart and Lisa agree to a criss-cross to get revenge on each other's teacher.
Don't Have a Cow, Mankind: An unholy new burger zombifies the town, but Bart is immune.
There's No Business Like Moe Business: A musical where Moe tries to win Marge with beer containing a dying Homer's blood.
05) The Devil Wears Nada
Marge gets turned on from her own sexy photos on a calender, but Homer is so worn out as Carl's new assistant Marge can't get him to snuggle with her.
06) Pranks and Greens
Bart learns about an even better prankster than him, but then learns that kid has since grown up to be a loser. Also, Marge succumbs to peer pressure from other mothers.
07) Rednecks and Broomsticks
After the Simpsons crash their car near Cletus' place, Homer becomes a moonshine taster while Lisa takes an interest in group of young women posing as Wiccans.
08) O Brother, Where Bart Thou?
Bart becomes jealous of Lisa's bond with her sister Maggie, and wishes for a brother he could bond with. Eventually, he settles with an orphaned boy.
09) Thursdays with Abie
Grampa's stories take notice with a human interest journalist who publishes the old man's stories, making Homer jealous.
10) Once Upon a Time in Springfield
Krusty is being phased out of his own show by Princess Penelope, who later admits she's madly in love with her childhood icon.
11) Million Dollar Maybe
Homer bails on going to a wedding with Marge so that he can buy a lottery ticket on a lucky day. Getting hurt shortly afterwards, Homer wins a million dollars but can't tell Marge or she'll learn why he bailed on the wedding.
12) Boy Meets Curl
Homer and Marge get involved in curling and while Homer is merely average, Marge's sweeping skills are so good they get to curl in the 2010 Winter Olympics.
13) The Color Yellow
Lisa digs up info about an ancestor named Eliza Simpson who supposedly helped free a slave named Virgil.
14) Postcards from the Wedge
Bart is forced to make up a month's worth of missed homework. Homer tries to force him to do it all, but Marge is more lenient, a wedge Bart is more than willing to exploit.
15) Stealing First Base
Bart tries to woo another (bipolar) girl in his life by following Grampa's advice: stealing a kiss from her. Meanwhile Lisa gets help being both popular and smart from a special guest star!
16) The Greatest Story Ever D'ohed
Flanders tries one last time to save Homer's soul by taking him to Jerusalem to soak in all of the religion.
17) American History X-cellent
Mr. Burns is imprisoned for grand theft and while he survives his new life there, Smithers takes over as head of the nuclear plant.
18) Chief of Hearts
Chief Wiggum and Homer become really good friends, but after Wiggum gets shot to save Homer's life, their friendship hits a bumpy road.
19) The Squirt and the Whale
A strong windstorm washes a whale ashore. Lisa tries to save it despite the odds.
20) To Surveil with Love
After a terrorist scare, Springfield gets cameras installed all over the place, and Ned Flanders eventually becomes the one to snoop on everybody. Also, Lisa is discredited because she's blonde.
21) Moe Letter Blues
As Homer, Apu and Reverend Lovejoy take their kids to an island park for Mother's Day, Moe sends them a letter stating he's leaving town with one of their wives.
22) The Bob Next Door
A new neighbor moves in which, to Bart's horror, sounds exactly like Sideshow Bob but looks nothing like him. What's the deal with that?
23) Judge Me Tender
Moe learns he's a natural at being a mean judge, and gets so good at it he goes to Hollywood to judge for American Idol. With Moe's Tavern closed, Homer annoys Marge with his presence.