I've heard a lot about this 'cool' feature in the next release and I wonder what it is. Any infos?
Light portals are a method of reducing noise, which, as you already noted, is new in 2.75.
Light portals help to remove noise by guiding rays from the environment through the openings where a portal is positioned. Because of the fact that they are designed for openings, they are most useful for architectural scenes.
It acts a bit like setting a light or HDR set to multiple importance.
Sampling is also slower when using portals, but usually the reduced noise more than makes up for the time lost while sampling.
To add a light portal, add an area lamp, and position it in the center of an opening. Make sure that it is facing inside. Then, using the lamp settings, change the dimensions of the lamp to fit the opening, and check the portal checkbox
Here are two comparisons from the docs:
It is a plane that defines where the most of environment light will come into the scene, so the renderer can sample light more efficiently.
Usually speeds up indoor rendering. The sample takes longer time but the noise will clear out after fewer samples.
Multiple light portals can be used, but it's only good to place them on the main light sources - like windows (or a TV if it's the main source of light in a night scene).
(Light) Portal is a setting for
Area Lamps that helps sample enviroment light more efficiently in Cycles. It's useful for interiors where enviroment light is coming through a window or door. You can get a clean image with less samples, although each sample takes a little more to render.
The end result is a small to mid speed increase, it depends on your scene.
In my experience they also seem to change the lighting subtly, giving it more of a "direction".