Pros:
Pretty good place to find many games, good deals in winter and summer and every blue moon. The software has lots of uses from organizing clans, has forums, profiles, and a little market to sell pixels for fake money to buy games or more pixels. Better than Trojanitor(origin). Implemented steam guard recently to help protect your account. Greenlight helps indie devs get their game on steam.
Cons:
It's community hammer legions are overly entitled and selfish. I don't like how you don't own the games and you are just paying for a license for life, if Valve were to go out of business one day then steam would die with it and there goes all your games. Also if someone hacks your account and gets you VAC banned Valve has a policy stating that they won't unban you and you will not be able to play on VAC servers for life. The screening team is somewhat weak, which increases the chance of a scam game making it's way onto the market. Also no refunds or the ability to trade games.
If you ask me the Pros out way the cons.
Poll