The only things I can think of are:
1) Upload bandwidth
2) Download bandwidth,
3) Maximum Download Amount per month (i.e. 60GB), and
4) cost to buy or RENT the hardware (Modem/Router device)
5) they probably also offer software like ANTIVIRUS but it may or may not be worth it. You are probably better off shopping around for a yearly or even 3-year subscription plan (Google for "best" AV software) though you may already have what you want anyway.
There will be different plans, and some of them should offer NO CAP (Unlimited download) on the amount, but if not then you will be charged a penalty for every GB you download over the limit.
You want at bare minimum 1MBps (8Mbps) download for Netflix. Upload is you sending data to the Internet so if you UPLOAD video in real time (i.e. game streaming) you'll want probably at minimum 300KBps (three hundred Kilo Bytes per second).
OTHER:
I believe it's at least 2GB per hour hour for Netflix. E-mails don't matter much, so it's mostly the following:
a) Game downloads (many games are now over 20GB... the biggest are over 100GB each)
b) Netflix streaming
c) Youtube streaming
d) general browsing, and MMO games would be relatively minor