These are my top attributes for tree superstardom:
1. The right tree in the right place.
It's ultra-important to get the right tree for the location and conditions. The exception to this is to put a spectacular tree where it shouldn't be, as long as you're willing to give it a lot of attention, and top-notch attention at that.
2. Take care of it from day one.
The moment you plant the tree, you should be doing some pruning and planning. But don't do anything unless you know exactly why to do it.
3. The tree should look good in all seasons.
Not all trees do, but most will look pretty good in winter if they're pruned correctly. Any tree that looks terrible in winter has been abused. Abuse can also include neglect, which is probably better than active abuse.
4. Girth, taper, proportion.
Except for some very tall forest trees that grow very straight, the trunk should have a noticeable taper, as should most of the branches. This not only looks a lot better, but provides strength and wind resistance where needed. A thick trunk makes for a sturdy tree and almost always makes a tree look better. A taper within all components takes skill to achieve.
5. Avoid whorls.
Whorls are the condition in which the tree splits from one trunk or branch into more than two branches at one point. Except for a few species that stubbornly grow this way, this is a big no-no, but almost everyone is clueless about this except for bonsai people. You'll see on almost every tree that at about 6' off the ground, someone decided to prune it in such a way that a bunch of branches come out of the trunk in one spot. Not good, but once the tree matures a bit, you're stuck with it and have to make the best of it. Doing it right from the very beginning is a lot better, and no, trees don't usually do this on their own. They would generally have staggered branches, which makes the tree better in every way in most cases.