How do you exit Dutch railway stations with an international paper-based ticket?

These barriers have already been in place at some stations for some time. In the Netherlands, paper tickets (e.g. from the ticket machines) will be phased out between now and July 9. People who don't have an OV-chipkaart will then need to buy a disposable one and check in and out as well.

NS announced the whole system will gradually go live after that with a handful of stations in the summer and probably 22 further stations before the end of 2014 so we will see how it works in practice very soon. The final decision on when the barriers will be put into function at all barriered stations (currently 82 are planned to be equipped with barriers out of about 400 in total) is supposed to come later.

Regarding travellers with paper tickets from abroad, from the same webpage:

De internationale klanten kunnen voor het passeren van de poortjes gebruik maken van de barcode op hun vervoerbewijs. Aan die klanten die een vervoerbewijs hebben die (nog) niet voorzien is van een barcode zal een zogenaamde ‘key-card’ verstrekt worden. Dit is een ticket met een barcode, die dient als een sleutel voor de poortjes. Aan klanten van vervoerders die besloten vervoer aanbieden van en naar NS-stations met poortjes wordt een passende oplossing geboden. Dit kan zijn:

• NS verstrekt aan de organisator (digitaal) key-cards waarmee hun reizigers zelf de poortjes kunnen passeren
• Op het station is een medewerker van NS aanwezig die de klanten van de vervoerder begeleidt bij het passeren van de poortjes
• De Servicecentrale van NS opent op afstand een of meerdere poortjes.

So the mains idea is to rely on barcode/QR-codes (at least one barrier per exit should be able to read them), have staff present or open barriers remotely. The page also mentions “key cards” you could get together with your ticket but it's not entirely clear to me who would give one to you.

At the moment, NS is still experimenting with this because it also creates a number of issues for people with special rail cards. They have extensive information about that (in Dutch) but there is a bit that should be of interest to all travellers:

Waarbij er voorlopig nog minimaal één poortje per uitgang open blijft.

This means that for the time being there will always be at least one open barrier per exit.

2015 update: The Dutch railways have introduced a reader for the self printed tickets and all international paper tickets should have a code to activate those readers as well.

These are the readers for the printed tickets, which have a little window instead of the common touch field.

Some of the gates have the possibility to read QR-codes from printed e-tickets. I noticed international tickets sometimes have these as well. For now, indeed, there will be manned gates (as Relaxed mentions), but in the future I assume that the QR-code on an international ticket will open the gate for you. Maybe it does already now? (June 2014)

Too bad I should avoid "making statements based on on opinion", otherwise I would have happily added a couple paragraphs.

If there is no exit available to you, you can alway use the 'i' (for information) station next to the gates (shown on the right in the picture). After hitting the blue button, you should get in touch with a person stationed somewhere else, who should be able to help you.