Near instant news feeds, esoteric data, well formed financial information about companies (balance sheets, cash flow, etc). IM service with other users, live market data, the ability to trade through the platform, the list goes on.

Terminals cost so much becuase the quality of the data and features that they have cost a lot. For starters, there are not many APIs out there that will give you live equity, fixed income, options prices, etc. for free.