Mayo. 
A Thanksgiving recipe thread is a great idea, Baker.
Moving to CS.
Okay, I can get that you made your own cream of mushroom soup, and I applaud your industriousness, but how did you manage to condense it?
My favorites are mashed potatoes, dressing, gravy, and dark meat turkey. A good portion of my family is vegan though, and/or avoids foods from the nightshade family, so I don’t usually get those items just for myself. Doesn’t make or break the day though.
For those bemoaning the pedestrian green bean casserole, I made Alton Brown’s recipe a few years back and it was definitely elevated above the standard recipe. Ignore the fried onion topping mini-recipe-within-the-recipe he specifies and just go with the canned fried onions.
You got it in one. That’s the one thing I look forward to all year.
Turkey’s pretty important, too, but I’ve had a Thanksgiving meal without it. Chicken and ham can make up for it.
This is true. One year the family had Cornish hens, and ham is always good. But Thanksgiving without dressing might as well just be Sunday dinner.
Stuffing (or dressing or whatever you call it) is pretty important to me, but I’m flexible on the details. And I won’t speak against my grandma’s recipe, but I’ve also had quite a variety of other very good stuffings. There’s a lot of room for innovation there.
There’s absolutely nothing snobbish about preferring home-made pie to store-bought. Store-bought pies are almost always mediocre-to-awful. A pie fresh from a real bakery might be elevated to “not bad.”
Which brings me to my quintessential Thanksgiving necessity, which I never have any more because I’m the only one who likes it: home-made mincemeat pie with rum sauce. Sneer if you will, it is ambrosia with nectar to me.
Roddy
My mom makes these incredible sweet potato muffins. I stuff myself with those and everything else is secondary.
I discovered this year that, as I struggled to find a gluten free pumpkin pie and didn’t really want to bake it, pumpkin pie is the item.
But if I had been looking at a no-turkey meal I would have been bummed. I love turkey.
I miss my mom’s roasted root vegetables. When I make them, they just aren’t as good.
ETA: Gah. I read the thread just to say that I need canberries. They are the quintessential Thanksgiving food.
Stuffing. I will gladly skip appetizers, potatoes, and dessert just to save calories for stuffing.
Once upon a time, I was dating a guy from a very traditional Italian family. I went to his house for thanksgiving and there was a truly absurd amount of food. Even taking tiny servings of everything that got passed around (the only way to avoid a barrage of questions like, “Don’t you like it? Why are you starving yourself??”), there wasn’t room on the plate for the turkey, which went around last. Then I noticed…
No stuffing. 
Keep your manicotti and your fancy mashed potatoes and your roasted everything. I need stuffing!
My Big Four are turkey, stuffing, mashed potatoes, and gravy. I make the whole feast entirely from scratch, and every part of it is magnificent, but those are the four elements of Thanksgiving. Plus cranberry sauce. I don’t actually eat that, but it needs to be there or it isn’t Thanksgiving. The stuffing has become dressing over the years as I stopped stuffing the bird, but old habits die hard and stuffing shall remain its name.
For a lot of years, we had a get-together on the Friday befor Thanksgiving. It was a pot luck. Yes, there were many turkeys, hams, potatoes, pies and all. But one friend always brought his famous tri-tip. It cannot be beat, don’t even try. It is not Thanksgiving without it.
They still have the get-together, but I have moved away. There is no Thanksgiving anymore. I am sad.
Another vote for homemade, hand mashed mashed potatoes!
Kroger makes a pretty good pie tho. Nice thick tender crust and plenty of filling.
DH is from PA Dutch country and they call the stuffing/dressing “filling” too.
I am a huge dark meat turkey and stuffing person, but they’re not make or break. I adore my aunt’s stuffing, but I’ve had to train myself not to expect it, since she got out of habit making it when I was away in college and doesn’t always remember to make it now that I’m back home. And obviously she makes the best turkey possible and I love it, but we’ll have Turkey on Christmas and sometimes Easter too.
What really makes Thanksgiving dinner Thanksgiving dinner and not just any old holiday is my mom’s sweet potato recipe. It’s based on Pommes Anna, so it always makes me feel fancy (my name is Anna). The dish baked in a round cake pan or a tart pan, depending on what’s clean and easy to find in the cupboard. You want the deepest orange sweet potatoes (what some would call yams) possible. Slice them neither thick nor thin- about a 1/4 inch, but don’t beat yourself up about it. Melt a half stick of butter in a cup with a spout (I just use a Pyrex liquid measuring cup). Drizzle a little butter in the bottom of the pan. Make a fairly tightly packed layer of sweet potatoes, heavily pepper, lightly salt, and drizzle on butter. Make another layer, trying to cover the gaps from the first layer, same seasonings, but add fresh thyme here. (You don’t want it on the bottom layer because more of it will fall to the bottom and burn). Keep layering slices, butter, salt and pepper, and thyme until you get the pan not just full but a little bit overpacked. Ideally you’ll save the prettiest slices for the top. No one will mind if you don’t. Cover with foil, put another pan on top weighted with pie weights or sacrificial beans, and bake until nearly cooked through. Then remove the foil and whatnot and get some browning on the edges. You can bake this at whatever temperature the oven is at if you’re doing it in the turkey baking oven, as long as it’s not above about 425. You can make it ahead and eat it later. When I lived with my mom I’d make it during commercials in the Macy’s parade and bring it to my aunt’s house after the dog show for dinner.
(Side note: am I the only one that thinks the dog show is as important as the parade in the pantheon of T-Day TV?)
On the flip side, I did not see marshmallow’d sweet potatoes or green bean casserole until I went to college in Montana. I like green bean casserole (I’m easily won over by creamy mushy green food) but marshmallows on sweet potatoes is just no for me. That’ll be fun when I finally manage to spend a Thanksgiving with my husband’s parents.
Hot flaky rolls. Don’t even try to bring me ones made from a can that you split open with the back of a spoon. It’s too busy in a Thanksgiving kitchen to even think about making your own, but go to a real bakery and ask for several dozen made ahead of time that you can heat up at the last minute. Also make sure to differentiate between “dinner rolls” and these, which are more like pastry - think croissants. I’ve had rave compliments on my rolls, even though all I did was pop them in an oven for ten minutes.
Wild rice of some sort. Thanksgiving isn’t complete without wild rice. I thought it was a national staple, but I rarely see it unless I make it. My favorite dish is wild rice with mushrooms - usually shitake. Great stuff. My Grandmother introduced me to wild rice. Of course they had a cabin in Northern Minnesota where the stuff grew liberally along the lakeshores. Do they still make wild rice dishes in Minnesota? Anywhere else?