I've got an original Gist in copper. A full-size international cartridge (Pelikan Edelstein) just fits. If there's some variation in standard international long carts, it's possible some other brand's might not fit.
I've had two minor issues:
1) The cap threads have loosened up a bit over time. Sometimes the lid will be a tiny bit loose. Hasn't fallen off, so I don't think it's a major issue, but I do wish it would stay snug.
Improved threads sounds great. As long as they don't loosen up over time, a half-twist to remove the lid sounds awesome.
2) Installing the feed section in the grip is a problem on the original... the grip section is so thin that the grip will split and crack if you over tighten the feed section. The grip is also razor sharp. =)
The slightly longer pen body sounds great. The original copper is my go-to everyday pen. Replaced the Bock nib with a Nemosyne 0.6mm stub, and I use it for taking notes at work. Heavy bastard, but feels great in the hand.