For me, the crunch to determining content is always access. My own access is ....not unchallenging for larger items ...... and involves a ladder so very large items are out. Neither would I put up there anything which could spoil if it became damp or subject to eg water/condensation/large variations in temperature. (Even the best regulated of houses might have a roof leak and lofts can become very hot in summer and very cold in winter.)
I've always lived in older houses so I'm also mindful of dust and grit - Gawd knows where it comes from but settle on loft items it does.
I guess I wouldn't be keen on storing anything electrical up there or such things as valuable photos which might be affected by water in some way. No fire hazards goes without saying, even though you've a detached house.
You also need to go up there and check regularly - as you would for any storage. Just to see that things are OK. there are no signs of 'other occupants' and that your storage isn't masking a house problem. (Eg roof leak dripping onto something and being temporarily absorbed by it so that you don't know you have a probem.)
Oh - and keep a weather eye on the storage weight. Your beams may not be up to carrying the roof plus lots of heavy items. (Keep the gold bars on a a concrete ground floor if you have one. 
)
Otherwise, I'd be using it, in practice, as 'deep storage' - not for things you're likely to use more than a few times a year, for example. (You might find those vacuum bags handy for such a purpose. (I think they're supposed to seal up the contents?))