My laptop recently died (won't turn on at all, and flashes console lights) and the repair shop I took it to said the motherboard was completely fried: they couldn't just replace individual circuitry components on it, but said it would need a completely new mobo, which would run like $350 and not even worth the cost vs. just buying a new one.

The repair guy said cause was likely overheating. But this laptop is only 2.5 years old. For the last 8 months or so, I'd basically leave it on 24/7...didn't have it set to sleep/hibernate/nothing...just a screensaver running. But it was always on one of those USB cooling fans, which I thought would prevent any overheating problems.

Was leaving this thing on 24/7 just a recipe for disaster? Are USB cooling fans not good insurance against overheating?

I don't know that would cause something to blow out, but I'd strongly recommend against disabling sleep/hibernate. Remember that's just a small area with a lot of hot components, no need to stress them nonstop when you're not using it.

If you run it 24/7, you need to make sure that it has unimpeded cooling (that its exhausts are not obstructed) and do regular cleaning of the internals (dust WILL collect in there, especially with constant use -- you gotta open it up every couple of months and take a compressed air can to it). If you leave a laptop running around the clock and don't take care of it, you're gonna have a bad time.

In addition to all the stuff that others have said, the thermal paste that couples the CPU and GPU to the piping and fans that keeps it cool enough to operate normally will dry up under heavy use. On more than one occasion I have had friends tell me that their laptops were dying, upon which time I diagnosed them as overheating. The fix there was for me to crack open the case, apply some good thermal paste to all the parts that normally require it, and then close it back up. Presto, everything is fixed! In the most aggressive case I saw, there was massive dust on the inside and stuff had been getting so hot that small amounts of soot started to burn off of parts inside the laptop. In other words, it is possible that stuff could get fried.

generally thermal paste doesn't dry up unless it overheats. So overheating kind of compounds and continues to get worse as the thermal grease dries up more and more. Best to not let a laptop overheat in the first place by always using it on a solid surface (not a lap)

Just set your laptop to automatically shut down when it reaches an unsafe temperature...then you can leave it on all the time if you want to...although as mentioned, not sure why you would disable sleep/hibernate regardless. Do you leave all your lights and tvs on also?

Remove the battery if your going to run 24/7 off the mains.

No.