One otherwise potentially lifesaving medication that requires close attention to the expiration date is sublingual nitroglycerin tablets or sprays, because they rapidly lose their effectiveness thereafter, or if exposed to heat. Also, once the bottle is open, it is probably best to discard and replace it after six months, although some new research has questioned the latter recommendation.
Most medications, with some notable exceptions, just gradually lose their effectiveness over time.
If you take a lot of medications, there are online applications that allow you to list them all and check for any interactions between them.
If you get prescriptions from different doctors, make sure they each know what all of your doctors are prescribing for you, especially your primary care physician.
Pharmacists are a grossly underused medical resource, and they are usually quite happy to answer your questions and fully explain everything you need to know about how to safely use your medications.
Some medications need to be kept in their original container rather than in daily pill cases, especially if there is a desiccant in the bottle. When travelling, especially internationally or with controlled drugs, you should keep your medications in their original labeled bottle.