I "gotta" call you out on this one. The difference between "good" and "bad" species for bonsai is not because "tradition" says they are bad. Quite often seasoned, experienced bonsai hobbyists and pro practitioners have actually worked with that species or similar species with similar characteristics and have learned through years of hard work and failure to create a pleasing image that certain species are not good candidates for bonsai. It is NOT out of laziness that myself and other more experienced bonsai people make these "pronouncements". These judgements are out of actual, hard earned experience. If you struggle a decade or so with a cottonwood, you can, accurately extrapolate to having similar problems with a popular or similar growth habit species. You don't have to grow all the "Bad For Bonsai" species to recognize where one will have problems.
Be my guest, waste your time, go ahead and try to create something with a species deemed "bad" for bonsai. You really want a challenge, try to make something attractive while in flower out of a Catalpa. Or try a tulip poplar, Liriodendron tulipifera, that another good one to frustrate you. Eastern white pine, Pinus strobus, is another that is uniquely frustrating to work with. The small handful of good EWP bonsa prove the point, that the majority are really difficult to work with. There are real reasons, from hard learned experience that "we" try to steer new to the hobby to the species that submit more easily to bonsai culture techniques.
@SilentMouse - you are in Michigan, I collect trees from my family farm near South Haven Michigan. There are some great native Michigan species to use for bonsai that will work more easily than poplars. Though poplars are not "the worst". And it is true, a winter view of a poplar with its white bark is pleasing. But while you are out and about in Michigan, look for hornbeam, Carpinus caroliniana, also Acer rubrum, red maple, the local native gooseberries, Ribes, make interesting flowering and fruiting bonsai. The MI native hawthorns, Crataegus, are great for bonsai. The eastern hemlock, Tsuga canadensis, is a nice, very winter hardy MI native. And if you get to central "sand counties" north of Grand Rapids, there is the jack pine, Pinus banksiana, that is quite good for bonsai, as is American larch, Larix laricina. The wild plum, Prunus americana, is also good for bonsai. There are many, many species that are good, and while it is not my intent to tell you what you can grow and what you can't, in an effort to save you time and minimize frustration, I would steer clear of species with less than ideal reputations.
Oh, and any elm, native or introduced invasive, that grows well outdoors in Michigan will make decent bonsai. Even the American elm, with its large leaves, it turns out the leaves reduce nicely in bonsai cultivation.
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