In all seriousness, good trees for bonsai usually have small leaves, or short needles. Their branch pattern is capable of ramification, into fine branches. Good species have interesting bark, either very rough, or very smooth and attractive or exfoliating bark that reveals interesting colors. If it is a rough bark species, the bark develops on young trees, less than 20 years old, in an idealized bonsai. Interesting or beautiful flowers and or fruits or seed structures are an additional bonus. Good bonsai are forgiving in their horticultural requirements. If it is difficult to keep alive in a pot, it is not useful, no matter how beautiful.
In reality, very few species get high marks in all categories. Even the most commonly used species may be lacking in one area or another. Many species are grown to show their strong trait in a specific season, and then are more or less hidden in the back of the collection for the rest of the year. For example, wisteria and Ume, are exquisite in flower, once they have leaves, they are pretty rangy and not that attractive.
Trees that are poor choices for bonsai have strongly expressed "bad traits".
For example - walnut, Juglans - has large, long compound leaves, and very coarse branching. Trunk barks up nice, but autumn color is not very good, yellow at best, usually leaves are blotched with black & brown spots as they turn yellow in autumn. So at no point will a dwarfed walnut look particularly attractive. By the same token, staghorn sumac, Rhus typhina, has all the faults of walnut, except it has beautiful autumn colors. The leaves turn wonderful, brilliant shades of yellow, orange and vivid scarlet red to red-purple. There are some beautiful examples of Rhus used as bonsai. They are only displayed in full autumnal color, and otherwise are hidden out of view for the rest of the year. Actually most species of the genus Rhus can be used this way, as they all have good autumnal colors.
Maples, most of the maples have big leaves, and long leaf petioles. When put into bonsai culture most of the maples the leaf size does not reduce much. Or if the leaf size reduces, the length of the leaf petiole does not. I have seen both sugar maple and red maple, with acceptable smallish leaves, hanging way out in space on long leaf petioles. A weird look. The good news is there are a small handful of species that do have small leaves that reduce well in bonsai culture. So of the 200+ species of maples, 5 or 6 make decent bonsai. Among them are Acer palmatum, the Japanese maple, Acer circinatum, the vine maple, Acer ginnala, Amur Maple, Acer buergerianum, the trident maple and Acer pseudosieboldianum, the Korean maple. There are one or two more, but this is what comes to mind. The other 193 maples are difficult to bonsai.
For some species horticulture is the issue. I have yet to see a bristlecone pine that has been successfully kept in a bonsai pot, east of the Mississippi River for more than 10 years. Similarly Mountain hemlock, Tsuga mertensiana, I have not seen any that survive long term outside the Pacific Northwest. So for me in Chicago, both of these species would be difficult choices as bonsai. If I lived in Seattle, or Portland, Tsuga mertensiana would be an ideal choice. If I lived in Fairplay, Colorado, a little town at 9000 ft elevation, Bristlecone pine would be a good choice.
Also, there are some lovely tropical and sub-tropical trees that I can not grow. Ixora is a lovely shrub, but can only live outdoors in a small part of South Miami, and a few small areas of California. Otherwise it is a very popular bonsai in Malaysia. So summary is, horticulture matters. What grows easily in your climate is "good for bonsai", what species that are difficult to grow in your area is "bad for bonsai".
Some trees that are bad
Catalpa - most species have huge leaves, and rank, coarse growth, generally not a "bonsai species" this hold true for the majority of members of the Family Bignoniaceae
Walnut - Juglans - most walnut species, european or north american are lousy bonsai, big compound leaves, coarse branching. Similar issues with hickories, all members of genus Carya are difficult to bonsai.
I was intending to go on, but realized time is fleeting. I have to get to other things right now.
I know some of my “ Suggestions” may have come across snarky and/or Obnoxious.
I recently posted a few of mine and was surprised no one felt obligated to tell me "those aren't bonsai yet" or "It'll be 100 yrs! before that looks like anything" etc. lol
