Ranked in these QuestionsQuestion Ranking
Pro Steam sales
Steam has daily and weekend sales as well as publisher sales, though their biggest sales are their winter and summer sales. Steam is known for their deep discounts (discounts on games range from 10% to 95%) on games and has pushed many other stores to do the same seeing Amazon and other storefronts match their pricings when sales go up as well as hold their own seasonal sales.
Pro Cloud save games
Sharing save games between Mac and Windows machine is pretty nice though the game has to be a Steamworks game in order to have this feature.
Pro Support for cross-platform games
Most games that have a Mac and Linux version available can be found on Steam.
Pro Biggest digital distribution platform for games
Over 125 million users actively use Steam.
Pro Fast downloads
Steam has excellent servers and their download speeds tend to be the best out of any digital store online.
Pro Wide selection
There are a variety of games from indie to AAA. There is a curated list and even classic games.
Pro Gift games
Users can gift games to their friends, which is great for when there are sales as one friend can pick up games for the rest of his friends cheaply. There is often 4 packs of a single game that can be picked up to easily gift to 3 other friends as well.
Pro Offers refunds for games that haven't been played for more than 2 hours
If you stop playing a game within the first 2 hours, you can ask for a refund.
Pro Steam Workshop
Steam Workshop makes it easy to create and/or find mods and extra content for games that support it.
Pro Integrated VoIP client
Text based chat and VoIP chat are both included with your client. Of course this only works with people on the stream platform.
Pro Active community
The community frequently helps solve questions, review games, and make great content via Workshop.
Pro Steam Greenlight
Steam Greenlight is a great way for indie developers to get their games on the market without a big Kickstarter campaign.
AAA titles are very pricey, very often more expensive than at other online stores and sometimes brick & mortar ones.
Con Buggy client on Linux with little customer support
Linux support is generally overlooked. It is recommended to disable in-game overlay and to use a real browser to interact with steampowered's website. Basic library access works usually without a hitch.
Also do not expect real tech support on Linux. Forum questions of bugs often go unattended and unanswered.
Con Little support for older titles
Many older titles (10-15 years or more) are no longer supported and thus require much manual tinkering to run properly on windows 10, requiring digging though forums and trying many things. Compare that to GoG where these titles often run out-of-the-box.
Con Lots of DRM
Many games are not only steam drm protected but by another drm system. It just cries for something to go wrong.