Mustrum_Ridcully
Legend
The 15 minute adventure day is a result of both the game system and the play styles involved.
The play styles basically consists of two aspects:
1) You want a challenging fight. This means you need to use your best tactical options (spells, powers, feat abilities) and coordination to win the fight.
2) You have varied options and you have to figure out how to use them effectively.
Note that probably everyone can enjoy both aspects, but for the 15 minute adventuring day to become true, these probably also have to be some of your highest priorities in defining how you run (combat) encounters.
In 3E, some of the best options are spells - both in terms of variety and in terms of tactical options. Part of the effective use is casting all the buff times at the right time, which costs you your spell slot already before the first started in many cases. Another part is using higher level spells, because they are just very effective. You will dedicate a lot of lower spells to repeatable buffing spells. Overall, this leaves the party expending their lower level spells early, and their higher level spells every fight.
Of course you only do this because you fight higher level encounters (but if you didn't there was no challenge)
And this is probably one of the primary reason 4E introduced encounter powers - even if you're all out of the daily spells, you still have some interesting, varied and powerful tactical options to use with care. The only thing really stopping you at some point is the lack of healing surges. (In 3E, hit points basically renewed every encounter thanks to the famous Wands of Cure Light Wounds, and the only thing you cared was how many high level healing spells the Cleric had still available so you could survive against full attacks and critical hits.
The play styles basically consists of two aspects:
1) You want a challenging fight. This means you need to use your best tactical options (spells, powers, feat abilities) and coordination to win the fight.
2) You have varied options and you have to figure out how to use them effectively.
Note that probably everyone can enjoy both aspects, but for the 15 minute adventuring day to become true, these probably also have to be some of your highest priorities in defining how you run (combat) encounters.
In 3E, some of the best options are spells - both in terms of variety and in terms of tactical options. Part of the effective use is casting all the buff times at the right time, which costs you your spell slot already before the first started in many cases. Another part is using higher level spells, because they are just very effective. You will dedicate a lot of lower spells to repeatable buffing spells. Overall, this leaves the party expending their lower level spells early, and their higher level spells every fight.
Of course you only do this because you fight higher level encounters (but if you didn't there was no challenge)
And this is probably one of the primary reason 4E introduced encounter powers - even if you're all out of the daily spells, you still have some interesting, varied and powerful tactical options to use with care. The only thing really stopping you at some point is the lack of healing surges. (In 3E, hit points basically renewed every encounter thanks to the famous Wands of Cure Light Wounds, and the only thing you cared was how many high level healing spells the Cleric had still available so you could survive against full attacks and critical hits.