Things not to do when building your FPS:

Overloading buttons in Enemy Territory Quake Wars

Overloading buttons, – giving a button/key duel functionality. While this can sound ‘intuitive’, it leads to confusion, then frustration if the player must remember the mode the button is in. Infamous examples:

• Run toggle in Battlefield 2. Double pressing ‘W’ to run. The correct implementation is to use a key modifier, eg. while holding W press SHIFT.
• Different modes via the same key in Enemy Territory Quake Wars. Double pressing ‘5′ to enter deployment mode, a single press selects the repair tool. This is further aggravated as some player classes need only to press ‘5′ once to enter deployment mode! The solution is to have one key for deployment mode and a different key for the repair tool.
• Different weapons via the same key in CounterStrike. Solution: bind each weapon to an individual key.

Incorrect highlighting in multi-player.  Mark the opposing team members on the HUD in the first person not 3rd person. (The world revolves around the player.) A good rule of thumb is that from the players view the enemy is always marked in red. Infamous examples:

• In CounterStrike the Terrorists are ALWAYS marked in red, the Counter Terrorists are ALWAYS marked in blue. From the player’s view point, when the player changes team the opposing team is now marked in a different color from what the player is used to. Solution: always mark the opposing team in red.

Relying on HUD Feedback during combat. Don’t assume the player can see the HUD during combat. During this mode the player is very focused on their target and will not see messages or visual alerts on the HUD.  If you have vital information to communicate to the player during this mode, use sound for feedback. (Once the shooting starts the player won’t even see the taking damage feedback.)

• Example: ETQW overheat bar and stalker feedback

Right angles hinder the player's movement in the environment - an example from Enemy Territory Quake Wars

Objects interrupting flow in the level design. Objects should be clipped (an invisible wall added) to prevent catching the player or hindering the player’s movement. Examples:

• Handrails
• Door frames that jut out
• Indestructible twigs! If you need to block the player’s access to an area use an object that makes sense. e.g. a large tree or high berm.

Losing the player don’t let players be lost in the map this especially applies to multiplayer maps

Updates to this post: this post will be updated occasionally with more ‘to check’ items.