The Dutch are, famously, the tallest people in the world — a fact they seem quietly pleased about. There are around 17.9 million of them in a country roughly the size of Switzerland. Density is high, particularly in the Randstad conurbation (Amsterdam, Rotterdam, The Hague, Utrecht), where something like 40% of the population lives.
The country is more ethnically diverse than stereotypes suggest. Large communities of Surinamese, Turkish, Moroccan, and Indonesian descent have shaped Dutch cities, particularly Amsterdam and Rotterdam, over the past 50 years. Rotterdam elected its first mayor of Moroccan descent in 2008 and has been consistently more progressive on integration than many European cities. This diversity shows up most visibly in the food: the Dutch eat Indonesian rijsttafel, Surinamese roti, and Turkish bread as comfortably as they eat stamppot.
There's a regional divide that runs roughly along the great rivers. The North is traditionally Protestant, more reserved, and pragmatic. The South — North Brabant and Limburg — is Catholic, more sociable, and distinctly warmer in manner. The Frisians in the far north have their own language (Frisian, officially a second national language) and a fierce sense of identity.
The Dutch have a talent for languages. Most people under 60 speak very good English, and younger generations often speak German or French as well. They rarely wait for you to try Dutch before switching to English, which can be frustrating if you're trying to practice. Don't take it personally — they're being efficient, not rude.
Dutch directness is real. They say what they think, ask about your salary without embarrassment, and don't waste time on pleasantries. Visitors from more indirect cultures sometimes find this jarring. It is, once you get used to it, quite refreshing.