Depends and totally varies with country. NOte that f.e. in Scandinavia nearly all local/regional trains are only standard=2nd cl. In East-EUR often same and hardly any benefits.
THE very best country to opulantly use 1st cl is the UK: on IC-trains you get free food/drinks, much more space and you have also access to many nice lounges-also with fd&drk in most major stations.
IN DK you also get free cf/T/OJ in 1st/ICtrains of DSB.
In Austria there are nice lounges for 1st cl in all major stations. In DE they have these too, but YOU with just a pass cannot enter!!
Overnight trains-if they even have seats, generally NO 1st cl-then you must book sleeper. It is NOT true that you always must REServe for 1st-again this really depends per country.
In some countries use of 1st is hardly enforced and each&sundry sits there. In others its mostly the tipical govmt workers on job-trips who will sniff at those stinky USA-EUrail users (in the past these people could only buy 1st cl. passes and hence the general image)
If you really worry too much about money, then skip it.