1st class versus 2nd class
2nd class seats are the standard way to travel in Switzerland. Swiss trains are comfortable, so 2nd class is perfectly fine. 1st class is about 75% more expensive. For that, you get more spacious and more comfortable seats as well as larger windows. On average, there are less travelers in 1st class.
In various long distance trains, including the Glacier Express, 1st class passengers have the option to have a meal served at their place. In some other trains meal services are available to 2nd class passengers too, but the options may be more limited than in 1st class.
Availability of 1st class
1st class is available in nearly all trains. Mountain (cogwheel) trains usually only offer 2nd class seats. Buses, urban transport such as trams, and mountains transport such as cable cars also offer just one class. Boats mostly offer 1st and 2nd class sections.
Traveling 1st class with a 2nd class pass
Do you own a 2nd class pass, and do you want to travel in 1st class on occasional trips? Then you can buy an 'upgrade ticket' (in German: Klassenwechsel) at a railway station.