Ethereal creatures treat all terrain as open terrain for the purposes of movement. They may not finish their movement inside impassable terrain. They are never slowed by any special rule, spell or item that would otherwise reduce their movement or stop them from moving completely.
Models with rule have both Magical Attacks and Unstable. Ethereal models have a 2+ Ward Save against mundane attacks. However, they only have a 5+ Ward save against magical attacks. Ethereal creatures block line of sight normally and cannot see through anything that would block the line of sight of normal units. Characters that are not themselves Ethereal are not permitted to join units that are (even if they become temporarily Ethereal for some reason).