No Biochemical Barriers: Averted, mostly.
While there will be absolutely no Half Human Hybrids, or indeed Half Anything Hybrids running around, as I said back then, it can get a mite fuzzy:
Alien Food is Edible: Well, no, for the most part it isn’t, given differing biochemistries, chiralities, etc., etc. Even really simple molecules can be tricky. By and large, if you end up in a serious Interspecies Romance or Mixed Marriage, you are going to need two kitchens, or at least two refrigerators. That being said… with the help of a genetic engineer, some helpful symbiotic bacteria, some drugs, and a really good chef, it’s often possible to find a few dishes you can share. Unless, y’know, one of you is made of rock or something.
Cross-Species Diseases: Uncommon, since for one thing, viruses just won’t work, and parasites often need to interact enough with their hosts’ biology to not work either. Bacteria – well, they’re more often a problem since the environment can offer enough of the right stuff to let them grow, the resilient and resourceful little buggers that they are, even if it’s not exactly the same disease when it happens to a different host, and immune responses can vary. (They tend to be much more of an ecological problem than a medical one, which is why they sterilize you – no, not like that – when you go offworld.) And allergen issues are downright common.
But anything cross-species that’s effective, similar, and contagious… that’s bioweapon/nanoweapon sign.
All Atmospheres Are Equal: Elemental abundances, energy sources, and the problem of Too Much Oxygen Causing Wildfires mean that there are a decent number of oxygen-nitrogen garden worlds out there, but the ammonia, halogen, hydrogen, methane, and sulphide breathers out there would like to point out that all atmospheres sure aren’t equal for them. And even the people for whom the basic gas mix is right still often need filter/compressor masks due to low partial pressures (or suits due to issues with the absolute pressure or temperature), toxic gases in the mix, and allergens. People who travel extensively often can get around some of these problems with extensive lung and immune system gene-work, but by no means all of them, and all the really different gas mixes/pressures/temperatures will still kill you.
Of course, all of these problems can be solved by doing the old Body Surf, but it’s not like that doesn’t have problems of its own…