Some guesses: Easily breathable atmosphere + liquid water + pleasant temperatures. Predictable weather? Mediterranean climate year round across whole planet? Entire world spanned by archipelagos, no continents (naturally condusive to smaller localised community building?) No native plants but rich soil for growing whatever plants we choose? Abundant sunlight with gentle constant breeze but few strong winds or storms? No volcanic or tectonic activity?
I'd say no pathogens but they'll come with us regardless.
I think NOT bringing the "right" microbes would be the problem, since it's not well understood. Childhood leukemia is linked to over cleanliness (lack of hepatitis A) [1]. Fecal transplants are being tested for neurological disorders, with some positive results!
Similarly, germ free rats have all sorts of problems, from under developed intestines to significant problems with brain development!
Who knows what's required, that we just happen to pick up while kids, sharing snot and mud with each other.
Tell me that when we can gene tailor hair back in our heads. When genetic therapy becomes a routine esthetic rather than edge medical procedure, we're almost there.