For most running, if you are comfortable in what you are wearing, then that's all you need. As Mike says, a lot of running clothing lasts well, and there isn't the need for this year's fashion.
For me, as I've got faster I've discovered I run hot. When I first started I could wear full length winter leggings in winter. Now, I wear full length lightweight leggings for training below 5C. I'd race in shorts in the snow, no problem. I'm a bit specialist because I sometimes run canicross (dog attached) so I need shorts that won't cause the belt (looks like a climbing harness, with leg loops) to give me a wedgie or thigh rub. As a result I use Element Karbon shorts which are mid-thigh lycra. Without the dog I usually wear standard running shorts.
The only stuff I have that's more specialist is:
Coat - I have several. Mostly I wear a windproof (not waterproof) to start on cold days. It usually comes off within 2 miles. Like Mike, I have an FRA compliant waterproof coat that I don't wear very much (bastard expensive and actually too hot to run in unless it's battering with rain).
Hydration pack - I have 2 with different capacities. I also don't carry water unless I'm going over about 15 miles, or it's really hot. On hot days I'll carry water for the dogs (although they mostly drink from streams). I mostly use these to carry other stuff. I do a lot of fell & mountain running, so the pack is to carry stuff to get me out of the shit - phone, vet-wrap (self adhesive bandages), foil blanket, extra clothes, dog carrying bag, my coat after the first mile. I have a Raidlight pack, which is super comfortable. Even full of stuff I don't really notice it.
Socks - I get blisters between my toes, so I use socks with toes
Gloves - despite generally being hot, I wear gloves when it's cold. I usually wear silk glove liners because it's about covering them from the wind more than anything else.
I think the main gain for certain types of expensive kit is being at the right temperature without being sweaty, and it being comfortable over longer distances. If you aren't uncomfortable, there's not a lot of reason to change. You may find that kit that is ok for 5 miles starts to be a little irritating at 20 miles. Equally, it may be fine.