Most trains are open seating and a seat reservation is not mandatory. Seat reservations are required for most high-speed trains such as TGV, Pendolino, Thalys, Eurostar, X2000, and highly recommended for ICE and certain InterCity and EuroCity trains. As Friday and Sunday afternoons and evenings are peak travel periods for Europeans, you may wish to consider a seat reservation. All long distance trains in Spain, Romania and most in Norway and Sweden require seat reservations. Many in Belgium, Netherlands, and Switzerland do not offer them.
First class is more quiet and spacious with fewer seats per compartment or car.
There is more room for luggage and generally will be less crowded as most Europeans using the train travel in second class. Depending on the route you use the answer could be no. If you wanted to go straight from Germany to Italy your train might go through Austria. It is an economical option for getting between major cities of Europe using modern coaches and making frequent stops. You make reservations for the bus in Europe.