Okay, maybe looking like humans isn't the rule. Maybe the rule is "****ability".
I don't find the idea of trills too repugnant. You're hosting millions of friendly bacteria yourself who help with your metabolism.
And, maybe being 'Evil' is an oversimplification. But look at the Trabe versus the Kazon. The Trabe are just as bad as the Kazon, but look at their roles. The Trabe are to the Kazon as 15th century Europeans are to 15th century Africans. And the Kazon are portrayed as wandering groups of warlords with crazy superstitions and fathers who kill their children for showing weakness. The Trabe are probably even worse than they are, but they're bad in a more 'Modern' way. And the Klingons are just as absurdly superstitious and irrational, even if for most of the modern series they're protagonists.
Romulans are TNG's cold war Communists, another 'Modern' evil, and a rational unsuperstitious one.
Same with Vorta vs Jem Hadar. Vorta are just as dangerous, but they're the rational manipulative evil, not the violent superstitious evil.
Less ****able = More superstitious, more tribal, more arbitrarily violent.