I regulary read complaints about the 5 Slot+Consumable+Ultimate bar. Most of the times people asume this has to do with the game being also desinged for the Console.
I think that is wrong. There is a totally different culptript for the limit:
The Action Combat System
In you MMO with more then 7 Slots of Quickbar you always have tab targetting/non-action combat. The reason is simple: Every number after 4 or 5 is incredibly hard to reach with your left hand while also keeping it on WASD. Unlike tab targetting games, you cannot simply unbind* the mouse in combat (hence this was put on a far off key). You need the mouse to look around and target. So nothing past those numbers would be reachable in combat.
In fact every MMO or game with Action Combat System will have a limit somewhat around what ESO has.
*most of the times you have to actively bind the mouse to change the view
In action combat systems you cannot take the left hand from it's position or the right from the mouse. If you do, you won't be able to react in time. That means the only buttons easily reachable are:
Everything on the Mouse (one good reason to upgrade past the 4 Button, 2 Axis Mouse)
WASD (movement), Q,E,R (often autorun), Tab, 1-5 (with 5 being already hard to reach), whatever is left of 1 (varries by keyboard layout), Space (often tied to jump), Shift (usually used to Sprint), Ctrl (hard to reach).
For non-comabt stuff there is a much wider array of Keys availible (even Ctrl and 5 are already at the border of combat reachable).
Other examples of very limited quickbars are:
DC Unvierse Online:

They choose a 7 Slot + Consumeable (Ultimates can be placed at leisure). Did not noticed that this might have been too many until now.
There is also a whole combo-system on the mouse buttons/light & heavy attacks (at least on par with what ESO does with Block, light/heavy attacks and Interrupt).
Neverwinter Online:

Is an adaption of the D&D 4.0 and 4.5 System. Right and Left Mousebuttons are you free/at will actions.
Q+E your resource limited/activations per combat limited abilities.
Tab enables the alternate combat mode (there is no way to lock a target in NWO)
2 Passives without need for any keybinds
1+2 are the ultimates. 3-5 are consumeables. 6 Is horse (already non-comabt area)
Wildstar:
wildstar-online.com/uk/news/wildstar_ui_20_preview_less_is_more.php/
Often considerd most direct competetion for ESO. Because it also uses action combat and it came out about the same time. They choose a hefty 8 + pet actions. Not sure how well that works out, but then again Engineer is worst case scenario. But I guess aiming withthe mouse makes everything past 5 unuseable in combat.
Counter Strike:
You got Knife, Pistol, Main Weapon, Grenades, Toolkit
I know that a First Person Shooter might be a slight break from the sequence, but it follows the same limits ESO has to follow.
Elana Peterson (EU), Dominion, Imperial Sorc, Rune & Alchemy Crafting Char
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