I have an E-Ticket by mail (they send it out after purchase online) and says "ETKT PASSENGER ITINERARY".

This is not considered a "paper ticket" is it?

If it is not, would I have to go to a Delta service Counter to get a regular ticket, should I want one?

Correct...it is just a printed confirmation of your trip. When you check-in, you actually don't even need it. Just give them your name & flight #, show your ID and they will print your BP.

IF you want to convert to 'paper', you would need to do it at a CTO or an airport counter.
From some of the info previously posted here, airlines (not sure about DL) are trying not to print paper tickets, and some have even levied a charge to switch from E-ticket to paper.

I've booked my last 6-7 trips via the web and never had a problem with the DL E-tickets.

They mail you the ETKT PASSENGER ITINERARY as a receipt. It helps the people checking you in, but there is no problem if you forget / lose / destroy that receipt since the ticket is in the system. This receipt can be re-printed anytime.

If you want a real ticket, you need to get that changed by DL.

I have been asked twice this year when checking-in if I had my ETKT receipt, both times the computers were down. So there are times when it helps to have the receipt.

I have flown around 140 flights this year. I don't think any of them were paper tickets. Never had a problem.

Yeah, I have never had problems checking in, and only once or twice been asked for the configmation number, etc.

I have seen references to people needed to switch airlines and needing a paper ticket and though that this receipt would not help.

When I made reservations this year in Bizelite to NRT, the reservationist tried to convince me that an e-ticket is much better than a paper ticket. The big benefit is that unlike a paper ticket, you can't lose an e-ticket. I listened to her pitch, then told her about the horror story I witnessed at FCO last year, while checking in for a flight to JFK. The gentleman in front of me at the Bizelite counter handed his passport to the counter agent, told him he had a reservation on the flight to JFK and was informed that no reservation was on record! She asked him for his record number...like most of us memorize it, then said she would issue him a new paper ticket at the full fare price. Needless to say he went ballistic! He asked for the supervisor but was told she is on break. I don't know what happened next as it was my turn. I breathed a sigh of relief when I went to the counter with my "paper" ticket and was processed with no problem.

She told me that was an isolated incident, the computers were probably down, etc., etc. I smiled and said thanks, but no thanks, and she issued me a paper ticket. On domestic segs, I use e-tickets but not on international segments. And, when using e-tickets I always keep my record number, just in case.