You can leave out these cards for sure:
Adventurer
Chancellor
Feast
Mine
Spy
Thief
Coppersmith
Great Hall
Scout
Secret Chamber
Torturer
Tribute
Torturer is the exact opposite of a beginner-friendly card, and the others are pretty weak and sometimes overrated by beginners. Then I suggest that out of the remaining cards, you just take the 27 that you personally like the most and I'm sure you'll be fine, as long as there are enough villages in there.
This is the approach I was leaning toward. Especially with Donald X saying that the base set contains some duds, I figured it would be best to start by removing all of those (plus some of the "torturous" attacks). Any others I should eliminate right off the bat? Woodcutter?
From there, I guess I should mainly try to make sure there's a good variety of abilities. Quoting one of the posts in the thread linked to by StrongRhino,
village
trash
gain
buy
attack (preferably hand-size)
draw
sift
possibly reaction...
lack of a completely dominant card
multiple cards at all price points
I'm thinking that it's okay to include a dominant card like Witch because I'm not just designing a specific 10-card set like that thread is talking about. There will be plenty of games without it so there's still going to be variety.
I'll try to work up a rough draft list later and post it here.