Here's some ideas for the Steam article
Several games available on Steam contain 3rd-party DRM. This is redundant, as Steam already controls access rights via login authentication. Most of these games contain activation limits, permitting play on only a fixed number of machines; subsequent installs will fail to activate.
This negates one of Steam's attractive features - portability. (That is, for most games, a user can install steam on any computer, log in, and play.)
Steam games containing 3rd-party DRM are generally available via other distribution channels. Developers may wish to avoid building special DRM-less versions for Steam in order to avoid additional effort and security vulnerability.
Steam has the following two concepts:
Users who are aware of the "Extra Copies" functionality from duplicate games in the Orange Box are unlikely to realize that Extra Copies are the exception, not the rule; most game bundle purchases will not result in giftable extra copies.
These issues are less steam-specific, and likely to occur with any digital distribution platform.
A user who purchases two games at retail may simultaneously play both on separate computers. A steam user may only play one at a time. This may sound silly, but becomes relevant if you'd like to let your roommate play your game.
A game purchased via Steam cannot be resold. In fairness, most popular retail PC games carry the same restriction via CD-Key activation.
Steam automatically updates games, and will not permit the user to play a game that is not updated. This can be an issue when updates break things, or if the user wishes to play immediately instead of waiting for an update.
Steam is only available on Windows; this restriction also applies to the games it delivers. Though several games offered on steam are available elsewhere for MacOS and Linux, Steam offers only the Windows versions.