The design of the Time Capsule case is very poor with no ventilation and a incorrectly placed fan which doesn't cool anything when it eventually starts spinning. The heat build up on earlier models which came with standard hard drives results in the capacitors in the power supply overheating and swelling over time causing it to eventually fail.
Heat/power was also a major issue for the ATV1, and was solved beautifully by going over to A4/iOS and removing the HDD. I can see a similar path for the next Airport Extreme as a possibility, but the TC requires an HDD, no?
B