I also saw your spool holder too. There are a few around now, I do like yours
On a more 'on-topic' note, I do agree about the rails too. However, I sort of like the plastic side of this printer. After all, the reprap scene does only exist because of self replicating printers which I'm fully in support of. So, I would say that this improvement should be done along side of the original design and not just replace it in the definition of this printer. Basically, what I'm saying is that while it is an upgrade, each plastic part that is replaced is moving the printer away from the reprap scene itself. So, if it were me (this is what i'll be doing in the future) I would make a completely separate machine implementing what you deem to be upgrades that remove the 'self-made' parts with factory/mass produced ones.
Once I have my printer completely setup properly and calibrating perfectly to my liking, I will be joining in with attempting some possible upgrades to this machine. The first one for me is going to be replacing the MK7 drive setup, I'm going to implement a MK8 drive and therefor the extruder carriage needs a little work done to it for the filament to flow through in the correct line of path without straining the motor or the plastic of the carriage itself. The reason why I'm planning this? because I use a fair bit of low temp PLA (170 degrees, ish) and this seems to need slightly more force/torque to push the filament through properly.