Noooooooooooooooooooo! Don't scare them away please!
I agree with those who mentioned that newbies should hear more accesible albums first. They may represent the genre well but scaring someone of doesn't help.
I mean, if you wanted to get someone into clasical music you woudn't give her Bruckner, Strawinsky or atonal music first. Starting her on Bolero and Carmina Burana would make more sense.
Similarly some prog-related stuff or 'easy albums' should be a good starting point for a future prog-lover. It's also important to pay attention to their tastes, of course (rock or folk?).
Good transition albums for the old stuff are IMO:
Kate Bush - The Kick Inside
Supertramp - Crime Of The Century
Moody Blues - This Is The Moody Blues
Deep Purple - Shades Of Deep Purple
Jethro Tull - Aqualung (anything but Thick... or Passion Play really)
Alan Parson's Project - Tales Of Mystery And Imagination
Mike Oldfield - Five Miles Out
Queen - Queen II
Rush - Moving Pictures
Fairport Convention - What We Did On Our Holidays
Steely Dan - Pretzel Logic
For newer stuff I'd think:
Green Carnation - Acoustic Verses (or anything with short songs)
The Decemberists - Hazards or Crane Wife
Dead Can Dance - Spiritchaser
Radiohead - Ok Computer
Gazpacho - Bravo
Motorpsycho and Jaga Jazzist Horns - In The Fishtank
Phidaux - Doomsday Afternoon
Neal Morse - ?
Agalloch - The Mantle
Therion - Lemuria/Sirius B
(for those ok with metal)
We are going to convert them all!