Some thrift store shoppers are collectors, others shop there because of the cheap prices, but nobody will buy an item in disrepair or which has no useful utility. Generally speaking most items that are encourages to be donated are furniture, lamps, books, magazines, CDs, DVDs, LP Records (my favorite), dishes, glassware, pots & pans, toys, bedding, working electronics, tools, etc. What are usually not considered a good donation are items such as chemicals, large older TVs, large appliances, baby furniture (high chairs, strollers, etc), tires, construction materials, pianos. Basically any item that takes up a large amount of floor space and has a small chance of being sold is not encouraged to be brought to be donated.
There are certain stores such as Habitat for Humanity which only accept construction materials and also help low income families purchase homes. It really is one of the best charitable organizations around. You should generally speak with employees at the store and ask what types of items they are currently looking for. Certain times of the year, clothing and blankets might be needed to fill demand during cold weather. During warm weather it might be bikinis and glassware :)
Please also be aware that employees and volunteers at thrift stores do have other things to do also, so don't bring your donations 2 minutes before they close the doors because everyone wants to go home on time. Just being courteous to others should be enough.