One of the first worries, based on news media reports, is the Roe vs Wade decision. That is the decision that protects women's abortion rights.
Those not familiar with that case, see: https://en.wikipedia.org/wiki/Roe_v._Wade
To me, the most worrisome part is the fact that Supreme Court justices are appointed for life, which means a far right Supreme Court could easily outlast the Trump presidency for decades - and when legal challenges reach the Supreme Court, they can not only rule in favor of the far right, but could also overturn any previous rulings the court ever made.
If Ginsburg also retires or passes on during Trump's presidency, don't be surprised is most, if not all, of the advances of LGBT rights end up overturned too and don't expect any new advances either. I have a feeling whoever Trump appoints to the Supreme Court will be some asshole who makes Scalia look like a liberal.
Ready for police to start raiding gay bars and beating people again? Ready for same sex marriages to prohibited and current same sex marriages to be annulled? Ready to be arrested in your own bedroom for having gay sex? Ready to be excluded from certain jobs on the basis of being gay? Ready to live in fear? I don't think it's farfetched to believe those, and more possibilities, potentially could happen - and if you are gay and living in the USA, you better be prepared for it.
I'm definitely glad I'm living in Thailand. If I ever return to the USA I doubt I would even be able to recognize the country I grew up in.