A game's camera is the medium that acts as the player's entry into the game world. If a game's camera is clunky, difficult to control, visually unintuitive, or wholly obtrusive to the gameplay - no one will want to enter that digital world. The best way to deal with these problems is to design and playtest around those issues - make sure the camera movement is fluid, and if not ask if there is a conscious reason why it shouldn't be fluid; allow the player to easily have some control over the camera even if it is a 2D game; make sure the perspective of the camera is intuitive, and if it's not intuitive make sure that there is a conscious reason why it shouldn't be intuitive; and unless controlling the camera is one of the core mechanics of the game, the camera should never interfere with the gameplay.
Star Wars: Obi-Wan Edit
Star-Wars: Obi-Wan was set to be a hit video game – capitalizing on the biggest franchise in the world with one of its most iconic characters in the player’s control. Unfortunately, the controls were horrible, in particular controlling the player camera. For this reason, it made the game frustrating to play; the challenge wasn’t in the gameplay, it was in controlling Obi-Wan correctly. Personally, I put this game down after a couple levels in as a kid who was obsessed with Star Wars. Controlling the camera felt like controlling an extra set of limbs suddenly added to the human body. Coordination was difficult for movement, let alone using other verbs while moving/looking around. This is the most memorable example I have of a game ruined by a terrible camera.
Assassin's Creed II Edit
Assassin's Creed II's camera is notorious in switching camera angles when the player is climbing from place to place. The tomb levels became a frustrating task for many players when the camera would swing away from the direction the player was trying to go, and as a result they would fall or jump into a wall. They would have to start the climb all over again. Assassin's Creed II's gameplay is based on the smoothness of the character's movement, and a jerking camera takes away from that experience.
The first installment of this franchise had some pretty terrible cameras. the cameras in this game were all fixed in position. Every room had a different camera that was positioned at a different angle. this made running from room to room very jumpy. The cameras also didn't play into the mechanics of the game very well. it was hard to aim at zombies when you weren't sure where your character was pointing. This was still great despite the bad cameras but could've been amazing if they were fixed.