1st Class gives you 2+1 seating and a wider seat, and more chance of a quieter coach and fewer young people in the coach. On Thalys you also get a free airline style meal and free WiFi and various other gimmicks. Thalys "Comfort 1" tends to be quite full whenever I see it. Look here for the difference between Thalys Comfort 1 and 2:
https://www.thalys.com/nl/en/services/on-board/comfort-1
https://www.thalys.com/nl/en/services/on-board/comfort-2