The team at iFixit has found that the Retina 5K iMac's display data cable is slightly wider, so it can handle the extra amount of pixels. The glass and the LCD on the display are fused together which might make replacing it harder. The display also has a number of parts from Texas Instruments, including its bus transceiver and its dual synchronous buck regulator. The motherboard in the new iMac contains an Intel Core i5-4690 processor and a AMD Radeon R9 M290X GPU. The GPU is soldered into the logic board so there's no upgrading that piece of hardware, but the CPU, hard drives and other parts can be upgraded and/or replaced.
It looks like the Retina 5K iMac is close to 2013's iMac in terms of construction. Be sure to check out our recent unboxing and hands-on of the new iMac. Are you planning to get the new iMac with Retina 5K display?