How well a character thinks and solves problems. This is probably the most important attribute because it determines which and how many skills a character may master. Certain skills can only be mastered by characters with an IQ of a certain level or above. Characters start with skill points equal to their IQ. During the course of the game, you'll definitely want to raise your IQ.
Intelligence represents the Ranger's ability to process and learn new information, and thus increase skills that much faster.
Intelligence is of primary importance to "brainiac" support characters, whether they be techies or medics, but should not be forgotten lest you find your Ranger unable to keep up with the team's progress.
Intelligence no longer determines which skills can be learned, but the amount of Skill Points gained per level. The sum of Strength, Speed, and Intelligence affects the number of action points.