Cheat: act dishonestly or unfairly in order to gain an advantage, especially in a game or examination
Feature: a distinctive attribute or aspect of something. (I'm not sure anything about DS is actually "distinctive" lol)
The temple exploit and the doubling of dailies (regardless of how you're doing it) are by the strictest definition, cheating.
People come down in different places on which exploits are acceptable and which are not.
Personally I don't have any feelings at all about the doubling of dailies. The effect on gameplay is pretty minimal and the modes affected are solo activities.
The temple exploit on the other hand disgusts me. (with a few exceptions)
DS is a team sport and temples are intended to be a team activity. To deny other members of your team shots at it is selfish . The only time I find it acceptable is if players are trying to catch up and bang out legendary quests OR if the guild as a group is trying to beat a record and everyone who wants to has already tried it.
Mail expirations are intentionally set by the devs. So to my mind it's no different than collecting stam packs (or even equipment) in inventory and waiting until the best time to use them.