Anything that isn't found in the suttas, but especially the tri-kaya.
As for the "why?"... because such concepts are not found in the suttas and much of it makes a mockery of what is actually found in the suttas.
... well, I don't reject anything not in the sutras, because there's a lot of zen and various Mahayana works that seem very well done to me, but I'm with you on the final point.
When first reading Buddhist texts, as I've explained, I didn't know the difference between Theravada and Mahayana and it took me a long time to work it out.
Actually, it was the different qualities of the texts that necessitated I do so - I was just reading eclectically and began to wonder why some sutras were well written and seemed to describe what I was experiencing and other sutras were full of sound and fury and sectarian attacks and poop that smelled of sandalwood etc.
It was that division in content that lead me to realise that the Pali Canon was a different order of texts (not getting into the argument about legitimacy or what came first). So I never chose a tradition, I came to investigate Buddhism as a whole and the Pali Canon spoke for itself in its claims on my attention.