Like most of you I'm sure...I have watched all the episodes...except 2 or 3...many many times. With the exception of perhaps a few seasons of All in the Family...I have never done this for any other show.
I essentially know the episodes through and through....but delight in anticipating scenes, watching certain shots...and picking up on a nuance here and there I hadn't before.....last night I watched Candidate for Crime for instance and noticed for the first time...Jackie Cooper touching Harry's coat (that he had switched and wore himself)..when Columbo first comes to the house ...Victoria's birthday party scene....he touches the coat presumably as a "loving" gesture towards his long time colleague.
I've also realized that I love many of the long shots in BY Dawns Early Light...they are highly effective...Columbo "in the distance" is very memorable I've noticed...when he walks into mess hall as an example.

So..my question is..what are a few of the subtle(or not so subtle) things you've picked up on after multiple viewings and/or what do you think keeps us coming back the most. How many of you have felt you've evolved as viewers of the show

I love Columbo's subtle way of telling the suspect that he knows it's them. Like when Columbo tells the killer that they'll be the first to know, and he has a particular tone in his voice. The absolute best example that I can think of is in Now You See Him when Columbo gets Santini to pick the lock during his show. Columbo says "I knew you could do it", which is basically saying now I know for sure it's you. And Santini knows that he just helped Columbo as well. It's that look on Columbo's face when he says it that is absolutely fantastic. These little things are what make Columbo so great!

I totally agree Irene...and there are many little moments like that...where Falk's acting makes these small tidbits stellar.
I love when Friend in Deed...Columbo is at Halperin's house, consoling him after the 'drowning" and says to the Commissioner...as he pats his chest..."you mustn't blame yourself sir"
I also love Columbo's facial reaction to being chewed out by Riley's lawyer in Publish or Persish...in Riley's driveway..."Evidence Lieutenant. find the evidence" etc...

absolutely, those little moments make the show very enjoyable to watch.

One of my favorites is in Prescription Murder, at the very end when the killer realizes Columbo's trick, he looks over at Columbo and Columbo gives him a look that I don't even know how to describe, but it's perfect.

There's no one thing that keeps me interested. It's a combination of things. Speaking primarily of the early episodes:

Terrific plots.
The singular atmosphere of the 70's.
The Americana.
The truly 'lived in' feel and contrast between scenery, from upscale and regal to the dark and gritty.
Realistic dialogue and nuanced performances from a variety of intriguing characters.
The warmer picture quality and cinematography.
The soundtracks.
And of course the man himself, Peter Falk.

Everything just worked brilliantly.