I'm looking to buy a NAS for the first time, I'm thinking something with 4-6 Bays with a price range of about $600.
I'll mainly be using it to store stuff and occasionally other people will access it to watch media.
They will all be accessing using laptops/computers so I don't think I need to worry about transcoding?
Edit: Should add that I have 2x4TB WD Reds doing nothing atm.
If you plan to leave it on all the time, get something with good software and a good power supply. Anything else will fail in one of millions of misery-laden ways. Every dollar less than a proper enterprise solution will bring with it a lower mean time to failure. The best consumer grade ones have lasted 10 years, are still supported and work very well. But they are slow by today's standards, and had bad power supplies that take them down early.
Most are unsupported after a couple of years and come with bug-laden, insecure software so could go to straight to the tip without passing Go, or a few years at your house where all that will really result is data loss and frustration.
Data loss has to be guarded against, and a big-ass-multi-GB NAS makes this harder in most cases. Just having RAID means zip and can make recovering data after a failure much harder.
Consider as part of the purchase a backup system/strategy to protect the data you store on the NAS as well as your PC.