Given my whole game design series began praising'decision points' - aka opportunity for player agency (decisions/tactics/choices) to influence the game - it seems unusual that I would decry having too many decisions.
But ever played a boardgame where you just know you'll be stressed and exhausted afterwards?
1.Games can have too many decisions (volume). Humans have a finite amount of concentration/willpower. We can focus well for about 20-25 minutes at a stretch. We have about 3hrs total of executive thinking/complex decision making time per day, which is very glucose-centric. (^Also food for thought about how long your wargame should last)
I'm not a fan of wargames where you can walk away for 20mins and come back and your opponent is still making his move - there's not enough engagement and the 'pacing' is out. But if both players are mentally exhausted by a constant stream of minute-by-minute critical decisions - well, that also might not be ideal.
If the stream of decisions is relentless - you can't look away for a minute, or have a casual chat with a mate while the game is progressing - then the game is only going to appeal to a particular audience.
2.Decisions can be serious/difficult or easy/inconsequential (impact). Not all decisions are equal in their stress level or consequences. Too many difficult decisions can be draining over sustained gameplay.
For example, in Infinity weapons fire is lethal and cover is very strong. Any opponent in LoS can react and fire on your active unit.You can continue to activate the same model, allowing you to chain together actions and perform deep flanking maneuvers. Each force might only have 5-10 models. Thus: a wrong move in LoS and you can lose a significant chunk of your force - or you could chain a series of inspired moves that allow you to outflank and rambo most of your opponents' force. Most Infinity decisions have serious consequences.
In contrast, in LoTR:SBG goblin bow fire might only hit on a 5+ and kill on a 6+ - only about 6% chance of death per bowmen shooting at you - and I might have 50 troops in my army. Moving troops in LoS might not be such a weighty, serious decision, both for me or my opponent. Similar is a complex/difficult decision - it's one where you may have complex if/then implications to juggle. Like Chess, where you are trying to look several moves ahead.
Whether it is the consequences or complexity - in the end it's about the 'brain drain'. Sometimes players need a 'breather' - or down time within the game.
3.Games can emphasize decisions in the wrong places. This is a bit different to the first two examples. Playtesting my "Forgotten" sci fi horror rules, I kept playing around with rules with initiative and reactions. Rather than using the usual alternate activation (each player taking turns moving a single mini) I slowly evolved it so players rolled an opposed dice every time they did something in LoS of an enemy. This roll did several things -
(a) determined the sequence of reactions/actions (and if reactions occurred at all, and ammo status)
(b) if the active player rolled below a target number, the initiative switched to the reacting player
It was actually a pretty good rule. A single roll that accomplished a few things at once, and lots of tension every time you activated a mini. Lots of potential decisions - not only what you now, but if you lose the initiative - then what?
This initiative phase was so gripping I was spending all my time checking angles and thinking about future implications/facing. Unfortunately I was so intent on this I kept forgetting a whole layer of the game - the demonic entities that could possess troops (a bit like Warmachine) and the horror/morale effects.
This mechanic would be great for a modern spec ops/SWAT game where the gunplay/angles/positioning was the point of the whole game. But for my horror game with psychic and sci fi tech elements they were the wrong choice - a set of serious decisions that drew attention away from what made it a unique horror game. I had a decent mechanic but it focuses decisions in the wrong place.
While points #1 and #2 were more focussed on the mental effort required by the player, this last point links more with the feel of the game. A contrasting example:
Warmachine is interested in the use of focus, synergies between units. It has IGOUGO which usually gives too much mental down time (aka disengagement) to the inactive player. I generally dislike IGOUGO but it is a good choice for Warmachine as otherwise it would be difficult to co-ordinate units. In contrast to my example above, it has a (usually) poor mechanic which suits the feel of the game.
TL:DR
A high volume of decisions or many difficult decisions (aka complex decisions or those with serious consequences) can place a high mental load on players. Other times, the wrong decisions can be emphasized - i.e. the players effort and attention is focussed on something that should not be central to the game, giving a completely different feel to the game than was intended.
Note: I may come back to this topic as I'm tired and feel I've only scratched the surface - so don't be startled if there's an edit adding in more points later, when I can articulate my thoughts better.