Long-running TV shows where the characters don't age.

"I Got Jiggy With It!"

"I gotta get on that internet"

George starting a computer company, with the cow-pattern Gateway boxes

Elaine semi-stalking JFK

High Fat yogurt affecting the 1993 NYC Mayoral Race, with Rudy Giuliani making a cameo(David Dinkins filmed one as well, in case he won)

Elaine being forced to see The English Patient

The entire episode "The Movie" is obsolete due to cell phones

Kramer getting a cell phone, and the entire novelty of it.

While there was no mention of national politics (Bush, Clinton, Gingrich), there were more then enough references to time stamp the show to the 90s.

 

A few you missed:

The dueling millennium parties

"This is the 90s, Jerry. It's Hammer Time!"

"You know who I saw wilding last night? Herb!"

The Simpson car chase and Harding shoelace parodies

Yeah, I guess there were more than I remembered. OK, I withdraw my nomination.

 

From what I understand, he was 40 for a time but for some reason it wasn't funny so he returned to being 39.

 

Yeah the implausibility of that was hard to overlook, especially toward the end.

 

It depends on the type of vampire. In the movie My Best Friend Is A Vampire they established that there are two types of vampires: Living and Dead. Dead vampires have to stay out of the sun while living vampires don't (also, they age at 1/10 the rate of normal people [to which the lead character exclaims "I'm going to be a teenager for 30 more years!?], and don't have to drink human blood to survive [pig's blood works quite nicely]) Also, when someone becomes a living vampire the others send him a guidance counselor to guide him on his new life.

 

To avoiding dating the series was the intentional reason that topical references were avoided on The Dick Van Dyke Show.