Probably the best (and easiest) way for me to describe Finnishness is to describe Finn’s mentality through stereotypes and values Finn’s appreciate.
Usually Finns are characterized as a shyish and quiet people, but that’s only part of the truth. Most Finns are modest, polite, honest and little abstained. Politeness is usually shown by actions and behavior instead of saying and small-talking is not a thing here.
I think one of the biggest factor that effects people’s mentality and attitude towards other in Finland is seasons: During those tiring dark seasons people usually like to spend time alone and are not so social, and on the bright seasons people wake up again, and are more outgoing and social. Most of the year is dark, so we make the most of these few bright months. Perhaps the grumpy and quiet Finn-stereotypes comes from people’s mentality during winter months.
Finnish people usually value nature greatly, and that’s why there is about 500,000 summer cottages and thousands of square kilometers of national parks here. Traditionally one of the biggest holidays, midsummer (Juhannus), is celebrated in summer cottages. During midsummer people head out to country to relax, go to sauna, eat and drink well and watch bonfire. Summer cottage is usually percieved as a place to “escape” living in cities, and to relax in the nature for a while.