System uses too much memory

Current linux kernels (if they were build properly) uses all available memory for required operations .

Very often when program terminates, the memory which was used by one process isn't cleaned, it still stored in buffers.

Thats the reason why you can see lots of memory usage, even if system doesn't do anything.

This isn't bad, because any other process could have same amount of memory (stored data in memory by first process will be overwritten by another process data).