Not from the US but from another country in the Americas. Here is a non-exhaustive list of things to take into consideration:
- Social life. Yes, modern technology makes it easier than ever to keep touch with friends from across the globe, but you need to expect to lose some relationships there. Not all friendships can survive the distance. The time zone difference also makes it annoying to keep in touch (e.g. you will be finishing a day's work and wanting to get in touch with friends, but their day has just started).
- Transportation. Now I don't know how you feel about cars but this was a huge win for me. I have driving and public transportation in Europe is usually very good. My driver's license has actually expired a few years back and I do not miss it.
- Compensation. You simply won't find jobs that pay as much as the ones you can find in Silicon Valley (or perhaps in the major American cities). This won't necessarily be all that base. See my next point;
- Reasonably good public services in general. This will of course vary between countries, but you can expect to find reasonably good public services all around Europe. Healthcare is included in this but it can sometimes be a bit cumbersome to get an appointment with the right kind of doctor (you need to first go through a general practitioner who will refer you to a speciality). This can be mitigated however. See next point;
- Cheaper healthcare in general. Even if you opt for the convenience of private healthcare, you can expect to find it waaaay cheaper than in the US. Your job with also offer you health insurance which will cover most of your expenses making having access to private healthcare really affordable.
- Job safety. I guess this varies with the state you live in, but in Europe you are likely to find that the labour laws work more in your favour than in the US. This can paradoxically get annoying for you in a few very particular situations, but all in all, it's nice to feel safe this way.
- Homesickness is real and there's not much you can do about it. You will long for your home, for that old group of friends you used to hang out with, for that spot you used to go when you needed some time for yourself. Also, seeing the people you left behind moving on with their lives will feel weird.
- You will gain a lot of safety. Now I have no idea where in the US you are, but I bet that a somewhat good neighbourhood in Europe will be safer. I came from a particularly dangerous place (doubt you will find a place as dangerous in the US) so this was a huge win for me.
- You will come to appreciate a different kind of life than what you are used to. I don't know how to explain this, but the way of life in Europe is very different from the one in the Americas. Life goals and such are all very different. This will of course vary from country to country.
- You will come to appreciate a lot of things about the US that you didn't appreciate before. I guess this is just a natural consequence of gaining perspective. Nowhere is perfect and nowhere is fully bad.