Being mixed-background Portuguese-British, and having lived here most of my life, I can actually weigh in on this.

The biggest no-no is the list of assumptions and biases you MAY carry on “What is Portugal”:

1. The assumption that Portugal is like Spain. There are similarities and there are major differences. We like our hermanos across the border, we share a History of rivalry and friendship, but that’s about it. 1000% they think the same about us. Saying “gracias” or “bueno” to someone will get you quickly dismissed with an awkward ‘whatever’ smile. Yeah, we understand Spanish (more than vice-versa), but it’s like going to the Netherlands and speaking German because they’re similar. The Dutch will probably understand, but the German will likely not.
2. The assumption that the culture/society you come from is far superior simply because - by comparison - living in Portugal is cheaper. I think this is the biggest mistake I see other Europeans (in particular British, German and Dutch) make. Portugal has fantastic road communication, telecoms and quality of life. By comparison to other European countries, we haven’t reached the level of “tourist-saturation” that you’ll find in Spain, France or Italy. Arguably, Lisbon is reaching that point, but still… it’s a small bit of the country.
3. The assumption that this is a “Mediterranean” country, and therefore you’ll have to haggle with the store owner to get a better price. No. Just no. This is an Atlantic country that grew up looking West rather than inward.
4. The assumption that people are lazy or dislike work. I can’t believe the number of times that I’ve heard the subtle undertone that people in Portugal are not hard-working. Regular office hours are 9am to 6pm. However, people tend to do more than their share and keep working till 6.30/7pm. On the other hand, I’ve heard too many people in Germany or the UK in meetings just hanging up at 5pm because it’s time to go.
5. The assumption that Portugal is like Brazil. Holy crap. No. Don’t make this mistake. This is not Brazil. Brazilian expats are all around the globe and they outnumber Portuguese when it comes to speaking Portuguese (and making a lot of noise), so it is a common misconception/assumption. Two completely different people, in both personality and motivations.
6. The assumption that this is a Catholic country, and therefore “conservative”. Politically (slightly) left-leaning, it is one of the most socially-liberal countries in the world. There is freedom of speech, of religion, of sexuality and even of drug consumption. It is one of the most tolerant societies on earth, when compared to other countries.
7. The assumption that the “downsides” are similar to any Latin American or African country. No. You won’t get assaulted by thugs, mobs or anything. Violent crime is almost unheard of (although domestic violence has made more headlines recently). You won’t get asked for a payoff / bribe on absolutely anything. Some form of corruption exists, for sure, but its more in the line of “jobs for the boys”, rather than hardcore corruption.

TL:DR leave the assumptions at the (faux) border and just come in and enjoy the country. It’s a melting pot of cultures and it is one of the most liberal societies in the world… by far.