I hope you folks realize that network half hour comedies in the 1960's were about 25 minutes of program, and only five minutes of commercials/ station breaks. Now they average 19 - 21 minutes of program, and 1-2 minutes less when aired on cable only or independent stations.
The first things to go are part of the end credits, (often a promo for another show on the same channel fills half of the end credits frame), the full opening credits/ theme song, cold openings prior to opening credits, recap of previous episode, preview of the next episode, subplots, epilogues, and non-PC jokes. Essentially, you're missing 20-25% of the show.
Sometimes time compression is used; the film is run at 26 frames per second, instead of 24, presenting each original broadcast minute in 55 seconds. If actors' voices seem higher pitched than they ought to be, time compression is usually the cause. TBS Superstation was notorious for compressing the early Sean Connery James Bonds movies, when they used to air them about once a month when they didn't have Atlanta Braves baseball games to broadcast at night.
In original broadcasts of Seinfeld, there was always a short epilogue scene between the final commercial break and the end credits. This is often cut entirely, or shortened, depend on which channel is broadcasting.
Sometimes the Canadian broadcast of a new network show is slightly shorter than the American version. With one hour dramas, there's often a 'previously on...' recap at the beginning, which is one extra 30 second commercial in Canada. Look for a new US show broadcast on CTV or Global, but not simulcast on Cable, and watch the first and last two minutes of each, and you'll see if you're missing anything.