I remain quite happy for now only to need to deal with one altimeter setting, and to have no exposure to the archaic TA concept.
I don't know how far your average trip is but before I had an IR, I had to constantly adjust the altimeter setting (QNH) during my longer trips, every time I was handed to a new ATC/FIS station (happens very often in densely populated Europe) and often in case it changes. Every airport reporting a QNH will be used by ATC/FIS to give you new settings and there are lots of such airports.
Now under IFR I set my takeoff QNH (airfield altitude at my VFR field), upon ATC contact I get the area QNH and once I pass 5000ft MSL, I'm at 1013hPa flying this value all across Europe until the approach. Much easier than constantly adjusting QNH.
I don't typically remain in contact with ATC on my VFR trips (up to 1000 miles or so). I set the altimeter occasionally as I pass airports and tune in their AWOS or ASOS to check surface winds etc in the area.
On local flights, I set the altimeter before takeoff and before landing: once on the ground and once in the air regardless of altitude or area of operation. However, if I transit through an airports Class D (there are a great many airports near my base) I will pick up their ATIS before calling them.
Well, VFR is VFR, as they say, and if you are visual then the exact altitude does not matter until you are about to land somewhere...
In fact your life expectancy will most definitely improve if you fly on random altimeter settings!
And if you are IFR (IMC) then you need to sort out your obstacle clearance pretty carefully otherwise you will die 
And flight levels (QNH=1013) are no good for that - unless you fly so high it cannot possibly matter, but that will also work for altitudes...
What exactly happens in the USA when one is flying IFR at say 16000ft? Does one periodically reset the altimeter to some ATC-issued altitude?
Knowing your exact altitude can matter quite a bit when VFR.... when climbing at 200 fpm to clear an 8,000 ft mountain :-) Nowadays I climb faster, but in the past I did some cross countries where that was a definite factor. If I need good altimeter data, I'll tune in some place and get it.
I believe the answer to your US IFR @ 16,000 ft question is 'yes' but others could provide a better answer.
What exactly happens in the USA when one is flying IFR at say 16000ft? Does one periodically reset the altimeter to some ATC-issued altitude?
On every initial contact with an enroute controller, a local altimeter setting in the area of the aircraft is provided.
Example:
Me: Atlanta Center, Bonanza 7083N, 16,000.
ATC: Atlanta Center, Bonanza 7083N, radar contact, Hickory altimeter 30.05.
Yes, if you fly in the US at e.g. 16000 feet on a cross country flight, you simply cannot expect to know how high you fly related to other traffic in your vicinity, unless you (a) both have the same QNH set or (b) both flight with the altimeter set to standard pressure. I think it is obvious that only the last option can work and that option (a) means that you have to reset your altimeter more often.
I suspect that if you recorded it, actually resetting the altimeter would be pretty infrequent for most en route GA operations. Nature doesn't know about ATC, or whether IFR traffic is passing between controllers.
OK, but my reading of the 18k proposal is that it won't actually affect many people, because the vast majority of UK GA flies low, say 2000ft or less, probably non-radio, and don't care for the QNH when enroute, and if you go a lot higher then you will soon be in Class A and then you need an IR and a Eurocontrol-filed IFR flight plan, and a periodic altimeter tweak is a non-issue. And almost nobody in Europe has an IR...
Peter, Notwithstanding the Class A airspace issue (it can't be everywhere above 2000 ft!) it seems to me the proposal would usefully eliminate altimeter hassle for those "2000 ft VFR guys" and encourage them to go higher and safer. If messing around with the altimeter going up and down to (say) 7000 ft were necessary, I would probably be motivated to say lower.
