A rogue has light armor proficiency, but I'm looking to get medium armor.
What is the drawback to using armor you aren't proficient with?
Each armor or shield has an Armor Check Penalty (or ACP), which is at least 0. The total ACP of your worn equipment is usually applied as a penalty to STR-based and DEX-based skill checks, such as Climb or Stealth.
If you aren't proficient with the armor you're wearing, then the ACP is also applied to your attack rolls (including combat maneuvers), STR ability checks, and DEX ability checks (including your initiative rolls). According to the core rules:
An armor check penalty applies to all Dex– and Strength-based skill checks. A character’s encumbrance may also incur an armor check penalty.
Shields: If a character is wearing armor and using a shield, both armor check penalties apply.
Nonproficient with Armor Worn: A character who wears armor and/or uses a shield with which he is not proficient takes the armor’s (and/or shield’s) armor check penalty on attack rolls as well as on all Dex– and Str-based ability and skill checks. The penalty for non-proficiency with armor stacks with the penalty for shields.
Note that there are exceptions, such as abilities that reduce an item's ACP. If the total ACP is 0, then these drawbacks are effectively negated, although arcane casters may still suffer spell failure chance.
(thanks to KRyan and Ifusaso for the clarification)