As overall, iMac had been quite durable. At least no less durable than your typical PCs. But in my experience there were a few components that more prone to failure/damage. Which are:
1. The screen: For some reason iMac LCDs fail more easily, possibly caused by excessive heat and airflow behind. It had several problems ranging from smudged clouding/dirty display (which I had several times), up to producing weird lines, colors, dead pixels, or won't turn up at all.
Those are normal lcd problems, but I can safely conclude iMac suffers higher risk to have it. If you're out of warranty/AppleCare, it's better to get external display instead. Way cheaper or costs the same as Apple's replacement, but you get another warranty with it. Plus iMac built-in display problems could come back anytime. And by that time comes again, you're screwed.
2. HDD: For same heat reason, iMac HDD tend to lives shorter than average PC's. I've had several PCs and it usually takes a long time (3 years or more) for my HDD to start failing and slow down. Some not even fail at all during ownership. But I've seen most of iMac in my office need HDD replacement just under 2 years.
3. GPU: A lot of failures happened on iMac with nVidia GPUs back then (pre 2009). Being a 4+ years old machines, Apple would cost you a ludicrous $300 - $500 for replacing those antiquated cards. Might as well buy a new machine instead. But statistics changed with Radeon chips since 2009 iMac. There were less failure reports since then.
And so I worried about next gen iMac GPUs which most likely will cycle back to nVidia. Maybe it's better to wait until Radeon again? But it could be 3 years, and it could be nVidia chips are getting more reliable nowadays? Nobody knows for sure.
Conclusion: iMac is a quite nice machine when its flaws ruled out, but I do feel it's not for me, too much failures and hassles for the price. LCD + hot components behind is not exactly a perfect combination. Yes, I'm a bit feel sorry to have purchased it. But lesson learned and I will hold on to this machine for several years, enjoy my AppleCare claims and when the time comes, I plan to switch back to regular tower + Macbook Air combo, and possibly with Apple display in the future.