I would imagine the 16-25 railcard is only valid for standard class because the perception of young people is that they don't know how to behave in the first class environment- where retired couples stare romantically out of the windows whilst supping on their complimentary sherry and the executives hammer away at their laptops and blackberrys.

I think it's mainly to try and keep first class 'exclusive'.
I do around 6-7 seven day ALR trips a year (I get 75% discount). I break the network up into parts, and over the week will complete that part, for example:
Part 1: Wales
Part 2: Scotland
Part 3: Midlands/London/South East (maybe 14-day version)
Part 4: North East
Part 5: South West
Part 6: Specific Trains (E.g. WAG Express, WSMR)
Part 7: Would be the NW, but as I live here and can generally get free travel, I tend not to spend a specific week doing this.
My "top tip" - Check the lines/stations that have very few trains a day/week/month etc, like Coombe Halt, Denton, Berney Arms, Ince & Elton and timetable around them so you can say you've been to these very difficult places.
I reckon from about 2012, I'll have the confidence to apply for my free FIP vouchers which will effectively give me a free unlimited all line rover for nearly every country in Europe. I'm still a bit weary of using these in the likes of Bosnia!