Let's see...
The battery pack/fan will die sometime between nominally 18 months to 48 months. The fan will get louder over time at the same rpm and will eventually seize. How long exactly though, this is a mystery. A rule of thumb is that fans that run all the time (apple portables) die faster. Smaller fans die faster since they need to run at higher rpm's for the same volume of air moved. Battery pack wise, the warranty is about 1 year for a battery pack on average, and Lithium ion batteries including polymer varieties reach the end of their lifespan at the rate of the fastest wearing down cell, which starts to degrade the day it was manufactured. Your air battery, if treated gently (properly maintained) will last about 350 full charge cycles while maintaining about 80% of your total initial capacity.
The SSD inside the air, while a solid state device, has a limited number of cycles it can run through. Statistically speaking a couple million cycles is what they are tested to endure. However, this needs to be under fixed voltage for writes, under certain temperature conditions, cooling rates, etc. And, this is a statistical figure only, meaning that invariably, some of your logic will have already gone bad and writes have been redirected to the spare sections by the time you have used your air for a few years. This happens without your knowledge, as all flash drives behave. Realistically speaking, SSD's are too new and constantly changing for reliability data to be gathered from non-accelerated testing (i.e. common use). HDD's on the other hand, have reliabilities quoted in the 10's to 100's of years, usually 50 to 70 years is common based on their mean time between failures model. However, I'm not aware of any drives surviving this long, seeing as how their spindle motors usually die out long before, its not usual to see a HDD in service beyond twice their rated warranty period. Other than that, seeing as how the longevity of a drive inversely affects the bottom line of a company, I'd assume the new SSD's will have similar orders of magnitude of service reliability as their HDD counter parts, not much more so.
Other than these components, the screen is LED back-lit, which has a life dependent on the driving voltage of the LED vs. the maximum voltage the LED's are designed to run at, and I have no clue what is the percentage used in LCD panels...
Other than these things, which are predictably going to fail, you may have some "gotcha's" like spontaneous failures of the GPU, seeing as how Nvidia have a bit of a reliability issue right now, and no one knows exactly what is going to happen.
Hope this addresses some of your concerns.
