The HUD communicates game concepts to players. (Theme, Progress, Abilities)
The HUD is primarily a way to convey information to the player about their situation and help them make informed decisions during the game. The HUD creates an immediate and long-lasting impression for players because it’s one of the first things they notice about the game when it begins and it never goes away. You can see how this could be a good thing or a bad thing depending on the quality of the HUD and how it’s perceived by players.
The first things the players notice about the HUD is aesthetic. Does it look good, and are the different elements placed in a pleasing way that doesn’t block the view of the action? Are there too many elements on-screen at once? Do all of the HUD elements have a similar look (color, shape, patterns), size and cohesiveness? These are the things players notice right away and are mainly associated with the theme of the game. You’re looking for consistency with the rest of the game. The quality of the art and the styling needs to match well with the game. Having A+ quality HUD elements with C-level graphics in the game is going to be jarring and feel weird to players.
The HUD Conveys progress and shows the player where they stand. Do they have enough points to finish the level with a winning score? Is their life bar full enough to survive the next wave? How much ammo is left to finish the next checkpoint? Is there any time left? These questions and more are what the HUD is for, and if you can hide HUD elements with the game, like the Halo series from Bungie putting ammo counts on the weapon itself. The Dead Space franchise by EA brilliantly used HUD elements that were blended into the game itself, like a life bar on the character’s suit, ammo count on the weapon, and even a “power bar” on the suit to gauge the amount of special abilities the players could use. It was all laid out like it belonged in the game anyway, and did not block the view of any action. That series is an impressive showcase of blending the HUD right into the gameplay.
The HUD is also a great way to tell the player they have access to a special ability, or how much of a special ability is left before needing to be recharged. Players use this information and the knowledge of how long it takes to recharge to gauge how much of the ability to use or if they can use any at all. This is critical in action or fighting games because the players must be aware of everything in their arsenal to make a split-second winning move.