I didn't really favor this aspect of 4e that much because I have always felt that, for the most part, magic items should be under the purview of the person running the game (of course this doesn't stop a player from having his character do in-game info gathering and seeking to find an item he is interested in). It's just my particular play style preference of course but I think even with artifacts, 4e went a step to far for me in placing magic items in the PHB and promoting a wish list and magic shop mentality (and no I am not claiming 3e didn't also promote some of this but you were speaking to 4e specifically).
Sure - I was just expressing that I came to broadly similar views about 13th Age as you did despite us having quite disparate views on previous editions' systems. The other points about 4E items are getting really off topic so I'll take that elsewhere.
I also think you're mistaken in characterizing the "true" magic items as MacGuffins since the definition of a MacGuffin is only a plot device with no further purpose, but "true" magic items mechanically benefit characters that have them...
This is a valid point, but I did say maguffins/
tools, and in the sense that they are a way for the GM to give the characters specific capabilities and "toys to play with" and see what their responses are, I would stand by this expanded description. To be clear, I don't regard it as any sort of bad thing to have such an artefact (as opposed to artifact!) in the game.
in fact once the player has a true magic item the only reason for the DM (and possibly the other players) to get involved is when the character decides to carry more true magic items than his level. Until that point I would have no problem with a player role playing out the minor actions and indicators of his items quirks and personalities that may arise since he is still in control of himself and his items.
Sure - I may not have been clear enough, but I assume that the player will be wholly responsible for expressing their character's items' quirks as long as they are within the "chakra limit" (horrible terminology, IMO, but I'll survive it

).
I can understand the desire for mechanical determination...and I might even consider using it, but I also feel like the player knew what he was getting into when he decided to take on so much power and that depending on the rolls he may not be inconvenienced often enough to make up for the added power of the extra magic item(s)...(I honestly don't know if this is or isn't the case but it's something I would be concerned about).
Yeah, I can see this concern as totally reasonable. It would be something to find out through play, I think.
The only reason I don't favor the player himself coming up with the effects is because I think most players will try to find an expression that inconveniences them the least, I mean they are the type of player who went for more magic items in the first place right? IMO, it's too much of a conflict of interest for me to feel comfortable leaving up to the player... especially because the extra magic item(s) are actually giving them increased power. I also don't want to spend 15-20 mins of game time with back and forth between the DM/other players and the overburdened player to determine an appropriate action.
My experience with Fiasco is that these determinations actually happen pretty quickly, but maybe that would work differently in 13th Age - I don't know. Fiasco has a similar system where a player leading the narration of a scene can have a "bad outcome" die imposed on them; they are then expected to lead the scene to a bad outcome for their character. If they offer an outcome that the other players, collectively, don't think is bad enough they just get an array of shaking heads/thumbs down to say "not bad enough, dude - make it worse!" If it worked that way with item influence in 13th Age I could see it working well and being fun; if for some reason it doesn't work out that way then I'd look for some other way to handle it.
I think it makes it more interesting and again more mysterious when part of the inherent danger of too many magic items is in not knowing what the exact effect will be once you put one too many on. But that's just me and I could totally see a game where the players and DM are comfortable with the player deciding, I think whichever option you go with probably gives a different feel and play dynamic to magic items.
Well, I would think/hope that players overburdening their characters with magic items will be relatively uncommon, but I guess we'll see when we start playing in earnest!