Loose Threads:

Who is Miss Barch's ex-husband? Could it be Rock and Roll Randy from "The Big House?"

Is Daria's example of reinforcement representative of herself?
(Note: this sounds like a reference, really it does, but I can't find it)

Andrea is in Daria's science class, but Jane is not.

Who is this Matthew that keeps calling Quinn? He's called at least once before ("Esteemsters"), and Quinn is always excited by his calls.

Would the vice president of the fashion club really wear pink with red?

Wasn't the Pigskin Channel only showing classic football games (i.e. games that are already over). So, why would the "game" be canceled? Or is Kevin really that stupid?

What is the name of Brittany's little brother?

How did Brittany get into the Morgendorfer's garage?

Notables:

The theme of this show ties in with a later episode from this season, "Pinch Sitter".

We find out about Brittany and the quarterback of the rival team.

Daria makes several pointed comments to Ms. Barch, Helen and Quinn about reinforcement, but none of them notice.

Quinn gets really nervous about Daria being included in Kevin's circle of friends.

Quinn makes a comment about not having to test perfume on bunnies anymore, something Sandi later contradicts (in "I Loathe a Parade") when she tells Tad Gupty that she does know makeup is tested on animals.

Helen's reaction to the phone ringing and Quinn's reaction to the doorbell ringing are both examples of the conditioning Daria was talking about, as well as possible direct references to Pavlov (someone Jane later mentions) who was able to train dogs to salivate when he rung a bell (they're indirect references in any case, as his name is the one most associated with conditioning).

Daria's description of the Pigskin Channel ("Great big guys slamming into other great big guys") is reminiscent of Terry Pratchett's description of the Discworld version of Sumo wrestlers.
"The wrestlers started to move hopefully towards the men, in a slow but deliberate run designed to be halted only by collision with another wrestler or a continent." -- Interesting Times
If Kevin were fatter, the rest of their description (one minor character failed to become a wrestler when he failed the intelligence test by failing to eat the table) would be fairly accurate.

Quinn hates having the tables turned on her; Kevin treats her somewhat the same as she treats guys.

Chuck's outfit in the garage is almost certainly modeled after one of Hugh Hefner's outfits.

Lawndale High's rival school is Oakwood.

Kevin has a really bad sense of direction. You have to wonder how many own-goals he scored before he got it right.

Quinn must be a good cook. Steak, cake, she's going all out for Kevin. We've never actually seen her in the kitchen, however, and you'd think getting all that icky stuff on her hands wouldn't be to her taste. Maybe she's just calling the good delivery services...

Dr. Pasteur was a chemist who had the gall to tell medical doctors they ought to wash their hands before they practiced surgery. His work against various infections (including Anthrax and rabies) managed to get him called a menace to science. Somehow, I don't think we need to hold our breath waiting for Kevin's greatness to be acknowledged.

Brittany carries a pink pager.

Charles collects spoons and dirty magazines.

Mistakes:

In the very beginning, it looks like Ms. Barch is using an overhead projector (it looks like there's a pull-down screen, and that is certainly not a chalk drawing unless she's got a lot of extra time on her hands), but in later shots, it's nowhere to be seen.

The dimensions of the Morgendorffer home don't really seem to be very stable (this one is confirmed by Glenn; there is no set floor-plan to the house as of the end of this season)

When Brittany confronts Kevin the garage for the first time, the maze and cage on the table disappear in some shots.

Joey, Jeffy and Jamie's voices are different than in "The Invitation".

Double Meanings:

Quinn asks Kevin how he liked his steak, and after some shouting at his team he replies "it's cool, and I really like this yellow dip."
The yellow dip is almost certainly mustard (or some variant), but Kevin is still wearing his yellow football uniform at the time...

Brittany and Quinn are talking at the door
Brittany:  Is Kevin here?
Quinn:  He can't talk right now, he's real busy with the maze thing
Kevin: (Yelling) Hey Quinn what's for dessert?
Quinn: (quickly shouting the door) Got to run,bye.
Brittany:  Ooh!

Old slang said that "dessert" was like a code for guys for "girl" after a dinner-date. As in "Did you have a good meal?" "It was O.K. but the dessert was wonderful!"