What to know when buying a "network-attached storage" (NAS)

So having recently jumped on the streaming bandwagon, I just learned about streaming media files using my Roku device. I have a 1TB external drive laying around and was going to use that temporarily until I got more storage when I learned about NAS devices. For those of you who are familiar with them, what are some key things I should look for when shopping around? I'll probably be the only one accessing it so I won't be running any kind of family shared network of some kind. I'd also like to access it remotely if that matters.

Oh yea, I forgot to mention that I read good things for the Synology DiskStation DS218+. Any opinions on that?

Buy something bigger than you think you'll need, storage wise, both bays and disks. You'll fill it up quicker than you'd imagine.

Are there any specific brands or models you would recommend? I'm only interested in streaming media to my tv.