Retraining...

[ARTHAN]

You can pick lower-level powers instead of higher-level ones, but you can't pick the same one several times unless explicitly stated (such as in Adroit Explorer). You can retrain feats into higher tier ones.

Allowing players to pick the same powers several times will tend to turn combat into them spamming the exact same move over and over, making the tactical experience more same-y and repetitive, and might lead to some degenerate optimization cases (it's basically Rangers always Twin Striking, except now everyone is doing it with their most potent encounter attack instead). I'd consider allowing it on a case-by-case basis, though. It could help out otherwise under-supported builds or players who struggle with the kind of tactical decision-making having a lot of different powers can require.

I don't see much reason to change the feat-retraining rules in this regard, there are enough bog-standard feat choices to fill out your heroic slots anyway so forcing people to keep heroic tier feats in all of them wouldn't really lead to any significant increase in variety anyway. I don't know what other purpose doing that would serve, either.

As a player, I love being allowed to choose the same power again and again because that, in the end, would had allowed me to focus in just 3 powers (an encounter, an utility and a daily) instead of turning my character to a mess of many different encounters, dailies and utilities. I was playing a player and DMing at the same time, so I had to allow that rule both as a player and a DM (I had too many creatures to command at the same time) and it turned out quite well actually (though my player character felt underpowered after a point).

I ought to also recommend it for 4th edition newcomers who can't handle a lot of different powers (or just don't want to) or vets who wish to build something really unique, like a "Sleep Wizard" or something.

As for the feats, I think there ought to be only heroic tier feats (with just a few exceptions maybe) and all paragon feats and all epic feats should be heroic feats instead, because right now it feels like "cheating" to replace a heroic feat with a paragon or an epic by using retraining and because you can use retrain only once per level up iirc.

EDIT: Just to make it sure, I disagree with the "Allowing players to pick the same powers several times will tend to turn combat into them spamming the exact same move over and over, making the tactical experience more same-y and repetitive, and might lead to some degenerate optimization cases (it's basically Rangers always Twin Striking, except now everyone is doing it with their most potent encounter attack instead)." part of yours. I have played that way (by having the same level 1 power 4 times per encounter) and it never felt repetitive, not as repetitive as you may imagine at least. Moreover, it wasn't like Twin Strike because I had it only for 4 rounds and only when it was useful; after that I had to rely on at-wills.