Yes, I'm disappointed that we didn't get 18-19 issues of T-Shirt and jeans Golden Age Superman. But that's not really a flaw in the writing, that's just preference/taste. Morrison clearly was never committed to doing that kind of Superman run. He's stated in interviews that this was about the bridge between that and the more DCU-conscious, fantastical Superman that we are more familiar with. Man torn between two worlds.
This Lex Luthor is just getting started. He's lived his life in a grounded reality before Superman, before aliens had overt contact with the world the way it happens in the first 8 issues of Action Comics. It's pretty clear Morrison based this Luthor on what Moore did with him in Swamp Thing. The genius consultant who comes up with the way to bring down the juggernaut. What's not to like? Furthermore, Superman #16 by Lobdell shows the state Luthor is in five years later, and he's more substantial than ever.
I don't know, I thought the use of the Fifth Dimension in Action Comics #15 was brilliant as hell. It's a different context from how it's done in RIP, which was a more grounded noir type of story and the nature of imagination/fifth dimension was left ambiguous enough that it still fits thematically with this version.