The battery is your primary concern - it is a consumable part. You may only get 300 to 500 complete charges before it will not take a useful charge. Additionally, as a Lithium Polymer battery with no adaptive charging, it fades every year once it's made. The lower the average temperature, the less it fades. Try to always keep it less than 75 degrees and time above 95 is exceptionally bad for long term health (think 20% loss of capacity in one year). Even if you store it perfectly, you can expect to lose 25% capacity within 3-4 years.
For long term storage, it's best to charge it to just above 50% and let it self discharge down to maybe 40 or 35% and then charge it back up to over 50. Having it always full isn't the best - but you also want to use it, so just assume you will pay for a battery sooner or later.
You have to decide - do you want a museum piece that is under perfect storage conditions (battery not too full and not too empty - both of which are bad)?
The extremes can damage a battery significantly - damaging the chemistry:
- totally low voltage - drained past all the low limits (leaving it in a drawer when discharged)
- excessive heat past 115 F is also very bad - even for a few hours
The rest of the failures are more about chance of random failure since you won't be dropping it or crushing it :-)
The hard drives in 4G and older iPods were designed for occasional use, so running them in disk mode or running a mac on an OS installed on the iPod was likely to cause them to fail after months or years of that use. You might avoid running an OS - but for normal syncs and play, the drive spins down most of the time and this shouldn't be a big concern. Physical shock when the hard drive is spinning would of course be bad.
I suppose the spring in the headphone jack would rather be out than in (some tiny amount of less stress) - but also less duty cycles is also good. I say, as long as you won't jam the plug / crush the internals - leaving it in all the time might be better than always putting it in/out/in/out.
You've got 5 years of swap service from Apple and you can always get a third party estimate if it ever dies, knowing that Apple will seal a new case in the factory. Any other repair (or DIY) can result in a bent/damaged or less than perfect case seal (As hard as it is to open them, the latest classics are even harder to close again properly than the old ones that you could use a nylon spudger or a flat razor blade to crack open.)
Good luck! I plan to get one of the last classics - once they go on sale as a discontinued product and treat it with kid gloves.