Miss 4e D&D

[Kurald Galain]

For example, let's say that you start with 3 stamina. 1st level Encounter powers cost 1 stamina, and 1st level Dailys cost 2. You get back a stamina with each short rest... but this might mean that you throw 3 encounter powers in a single fight, instead of saving for your daily. And as you level and get more Stamina, you might decide to KEEP your low-level encounter powers, rather than improve them... because, with 20 stamina, you can toss out 20 Encounter powers, rather than just a few.

That's interesting, but it risks having the same issue as psionic powers (i.e. that the best option is to spam a single low-level power all the time).

About "inventive play", it is in fact explicitly allowed and encouraged. The game never says powers only do their described mechanics and have no further effects,

It is interesting that you bring that up, because the prevalent opinion on this forum (and the WOTC forum, back when it existed) is that powers do exactly what their mechanics specify, no more, no less. This is also the opinion of pretty much every DM I've played with. For example, a Fireball spell works just fine in a heavy rainstorm, deals normal damage to fire elementals, will not set a forest on fire, and does not provide a bonus to Intimidate checks.

One is in the improvised attack rules, which are exclusively interested in determining damage by using a lookup table. You pick the level of the character doing the attack, determine if it's easy / medium / hard, and lookup the damage associated with that attack

Note that 4E's rules on improvised attacks ensure that in almost every case, an improvised attack is less effective than an at-will power. Obviously, this is an incentive against using improvised attacks.
Likewise, the DMG states that in a skill challenge, an original approach by a PC gets a higher DC than following the DM's suggestion. Although this was eventually errata'ed, it's also a clear incentive against improvising.