So, I took a look at 5e's basic rules via PDF. Nothing really jumped out at me as great or terrible - just different - until I hit the exhaustion rules. Once I read the rules, I went back and checked the authors, specifically rules development, and saw a familiar face from the brilliant (in my opinion) design team that did Star Wars Saga Edition (SWSE). The SWSE did one major thing that I felt was not-so-brilliant - one might so so far as to say game breaking, and it looks like it has made it into 5e - a death spiral Condition Track, in the form of the Exhaustion rules.
We had to House Rule extensively to remove this
completely horrible idea"feature" from our SWSE games, as it essentially gives each character and opponent (PC, NPC, and monster) around 6-7 effective Hit Points against certain builds.
Is this an optional rule or is it a standard rule? How common are effects which cause exhaustion, in the form of spells, class features, and so forth?
I don't have the PHB, but in the basic rules the only things that cause exhaustion are Forced Marches and lack of food and water. No spells. No abilities. Nothing else.So far I don't see any signs of a death spiral.