My baby doesn't smile very much - could this be a sign there's something wrong?

Maybe. Babies generally begin to intentionally smile between 6-8 weeks. Before this they tend to smile mainly in their sleep. However, remember that babies develop at different rates and there is a wide range of normal. If you are concerned about your baby's develop bring these concerns up to your pediatrician.
Maybe. Most children begin to smile responsively around 6-8 weeks of age. If your child is not doing so it could be a normal variation in development, their "personality", indication of a vision problem or sometimes a sign of developmental delay. Any time a parent has a concern about there child's development, their pediatrician should be consulted.