Thalys trains have 1st & 2nd class seats and café-bar. On Thalys, 1st class is marketed as Comfort 1, 2nd class as Comfort 2.
Should you go Confort 1 or Confort 2? Confort 2 (2nd class) is perfectly comfortable, there's absolutely no need to fork out for 1st class. But if you don't mind the extra cost, Confort 1 gets you more leg and elbow room as seats are arranged 2+1 across the car width instead of 2+2 as in Confort 2. That means that as well as tables for 4 and dual side-by-side seats, in Confort 1 you'll find tables for 2 and solo seats on one side of the aisle. A face-to-face table for two in Confort 1 is ideal for couples. And in Confort 1 you also get a meal or snack included in the fare, served at your seat, and can use the lounges at Amsterdam or Cologne.
Seats facing forwards: The train could enter service either way round, so it's not possible to specify forward-facing seats on Thalys as the reservation system doesn't know which seats will face which way. But this is really not a big deal on a smooth-riding train.
Fares & ticketing: . Like Eurostar, reservation is compulsory and all tickets include a seat reservation on a specific train. If you want to stop off, that means two separate tickets. Booking usually opens 92 days before departure. Thalys fares work like air fares, rising as the cheaper tickets are sold. For example, Paris to Amsterdam starts at €35 booked well in advance and rises to €130 for a fully-flexible ticket, so book as far ahead as you can!
Boarding Thalys: New security measures in place at Paris & Lille: As with most European trains, there's no check-in, you just walk into the station, look at the indicator boards to find your train and get on, any time before it leaves. Doors may be closed a minute or two before departure. From December 2015 in Paris & Lille there is a simple baggage X-ray, metal detector & passport check at the entrance to the platform. It should only take a couple of minutes, but when joining Thalys in Paris or Lille it's best to arrive at least 15-10 minutes before your train. It's largely security theatre to reassure passengers, there are no similar checks at other stations.