Frankly, I'm amazed at some of the advice here so I'm going to go for a big old post and then we'll all have a pint at the end.
First and foremost, first-aid is a misused term. Strictly, it's 'Simple emergency medical care procedures intended for lay rescuers to perform before emergency medical professionals are available.' This basically means "make sure they don't die". If all you want to do is make sure someone doesn't die, then sure, carry a few dressings, a yard of muslin and a mobile phone. Take a look at a WWII first aid kit, it really had naff all in it, even though the chances of a traumatic injury were far higher than we have out in the wilds. This is real "patch 'em up and ship 'em out" territory.
The next level up from that is "make sure they don't die and have the best chance of making a full recovery". Sure you can treat a burn with cold water, but if you've just slipped over and put your hand in a fire and you run that in a stream for a bit, then not only do you run the risk of getting lovely bacteria in your wound, but that wound is not going to heal anything like as good as if you applied a burn dressing or burn gel. Application of anti-septics and the use of gloved or at least clean hands would be awfully useful here too. If you want to cut plasters or tape with your knife that you've been using to cut all manner of stuff, then you go ahead, I'm going to carry scissors. I carry steristrips, because they give the best chance of a deep cut healing back to normal. I always carry a survival bag. My mate (mountain rescue) also carries a sleeping bag and bothi, even on a day trek.
This is basically where the RNLI first aid stops and we carry a first aid kit the size of a suitcase and are trained to use it. Yes, it's so big because we could come up against all sorts of problems, but we really are into keeping people alive and getting them to hospital or helicopters with the best chance of survival, not about trying to treat them at sea.
OK, so that's first aid, but in reality what we as campers, hikers, trekker and bushscrafters carry are 'medical kits'. We're not merely concerned with keeping people alive, we're interested in treating what we can. We don't want to have to take people to hospital or home if we can help it. If you've ever had a minor strain then you'll know it's actually quite painful unless you're strapped up when it's really quite ok. A good ankle strapping allowed me to finish a charity walk. I would have been gutted without that opportunity. Cuts, stings, strains, blisters, blood blisters, sun burn, ticks, splinters, minor pain and inflammation and other injuries can all be managed with very light weight kit. Yes, you can walk in pain for five days on a splinter and it go septic when you get home, but I'll carry tweezers. Yes, I can use my left thumb because my right one has blood under the nail, but a paper clip will sort that out. Yes, I can guess if someone has a fever, but a head strip thermometer weighs nothing. I'll take immodium rather than battle with diarrhoea and consequential dehydration.
Further along the line are things that just plain make people comfortable. If I'm trekking, I don't like guts ache, so I carry antacids. I don't like dry lips, so I carry lip balm and I don't like pain, so I carry pain killers.
I choose my first aid kit contents based on what I'm going to be doing, how long I'm going to be doing it for, the environment, the weather, how far I am from civilisation, who else I've got to look after and what level of care I want to give* and what I'm qualified to administer**.
* The level of care I want to give is generally. Don't die. Don't call the emergency services unless I have to, especially if they are voluntary. Get myself home (or back to civilisation or camp) if I can. Don't go home unless I need to. Be comfortable whilst I'm out. But equally, don't carry so much that I can't carry something else useful which I'm more likely to need or want. I don't carry a neck brace, for instance.
** Corollary: always take a nurse
There is a very big difference between a pocket kit for a day hike in along the coast path and a five day camping trip on Dartmoor with a group of school kids. I don't light fires when I'm hiking, so I don't carry burn stuff. None of the kids have angina, so I don't carry glyceryl trinitrate spray.
There is also a difference between what you have in a day sack and what you have at camp. You don't need to carry all of it all the time.
So the answer to the question "what belongs in first decent first aid kit" is all about what could go wrong and how much you want to let that spoil your trip. And the mere fact that it was asked with the qualifier "decent" implies it wants to be somewhat more than emergency patch up.
Remember, if you're in a party and someone needs to go home, they'll be taking someone perfectly healthy with them and if that can be avoided, then all the better for two people.
And finally, just because you have not injured yourself before and you consider yourself very careful, doesn't mean it won't happen to you or someone you're with. That is the nature of accidents. It can be high impact stuff, even if it's very low risk.
Now, who fancies a pint?