A scalable system should have the following characteristics:
You should gain proportional increase in performance when you increase the resources.
Your system should be resilient and fault tolerant. Your system should be recovery oriented. If there are failures it should recover from errors fast.
A scalable system should be operationally efficient.
A scalable system should implement heterogeneity due to the changing nature of hardware.
A Scalable system should be loose coupled and elastic.