I fly Delta a lot these days. What you need to know. Get the app or go on line and check your flight once a week. They USUALLY do adjustments to their schedules/aircraft on Saturday nights, so Sunday or Monday is a good day to check. That way, if something happens you can get on it right away and get your seats adjusted or whatever.
They DO NOT charge for seat assignments unless you are seeking some special seating. DO NOT choose "Basic Economy" fares because that comes with NO ability, at any price, to choose seats...those are literally assigned when you check in, and they tell you straight up that they do not guarantee that your group will be kept together, even families. Main Cabin fares do allow you to choose seats, and as long as you aren't seeking one of the favored rows or locations, you definitely can get those with no extra fee (although this fare does cost more than Basic Economy). Depending on the difference between fares, it can be worth it to do "main cabin." If you want extra room, choose "economy comfort" which also includes beverages and "upgraded" snack options. Seats are at the front of the cabin, right behind first class.
You will pay for luggage unless you have the Delta Amex card, in which case you get one free bag per person on your itinerary. Usually, the card is "free" for the first year....if there are a number of people in your party, it can be completely worthwhile to have the card. If you fly once per year, and want to check two bags (for a group of two, for example), you would save money over the annual fee. The card also gets you "Zone 1" boarding privileges....which isn't worth a whole lot because Zone 1 comes AFTER "preboards, Premium Cabin, and their frequent flyer "Diamond, Platinum and Gold," which in some cities can be literally 1/2 the plane. LOL.
Delta does have one of the best on time records going. And, because it is a major airline, they do a better job than most at dealing with problems and getting you to your location on an alternative flight. That's one reason I like them compared to some of the smaller companies. They recover quicker.
I like them well enough. I buy main cabin fares and then watch closely when I check on my flight status (see first paragraph) to see what I'm offered to "buy" a first class upgrade. They are aggressively trying to monetize first class and have decided they would rather "sell" those seats than offer them "free" to their mega frequent flyers...which makes the frequent flyers unhappy, but that's a "them" problem. Usually for a nominal amount, you will get offered the chance to buy your way into first class. I have a number in mind for each flight I take, and if the offer hits that amount, I buy. It's a significant step up in service, leg room and comfort, not to mention completely free bags (up to two per passenger, and they can weigh 70 pounds rather than 50). You will get offered your choice of beverage while on the ground, warm towel service, choice of full meals served on china, etc. Civilized.