What's in a 'Paella'?
Traditionally paella is a poor man's food, so what went in depended on what you had. If the only meat you had was snails, you made your paella with snails. So you'll find some people today insisting that to be truly authentic, paella valenciana should have snails.

The "anything goes" mentality still persists to some extent: it's not easy to find two Valencians who agree 100% on the recipe. And in addition to valenciana you have paella de mariscos, de verduras, and mixta (and should arròs negre count?) On that basis the defining aspects would be the type of rice and the technique: a flat pan and a long cooking time to extract flavour from the ingredients before the rice goes in.