I've got a bike which I bought secondhand a couple of months ago, before my trip to the Alps. It's turned out to be a cracker, not to mention a hell of a bargain - apart from one thing: Internal cable routing.
I've had this before for gear lines, but this bike takes it to another level with internal routing of the brake hoses too - and not just through the mainframe but the chainstay as well.
Having had to swap the brakes over in resort and zip tie the new one to the top tube, I've now added internal cables to my list of bike purchase deal-breakers.
This list also includes...
Press-fit BBs on MTBs
SRAM gearing on MTBs
Short warranties on expensive frames