If you add 'sex' to your list, you have a complete list (of the major factors that matter). So it's not like there are a whole lot of reasons.
"If you cannot prove you have any proper reason, a racist reason can be assumed and you're facing trouble."
No, this is not true. In the general case, the burden of proof is on the claimant, i.e. the person feeling discriminated against. There are specific circumstances where case law stipulates that discriminatory motifs may be assumed unless proven otherwise, but those are far and few in between, and to be interpreted narrowly.
"If you cannot prove you have any proper reason, a racist reason can be assumed and you're facing trouble."
No, this is not true. In the general case, the burden of proof is on the claimant, i.e. the person feeling discriminated against. There are specific circumstances where case law stipulates that discriminatory motifs may be assumed unless proven otherwise, but those are far and few in between, and to be interpreted narrowly.