For us was the general quality of design
and build. From past boats we owned, we knew we wanted a really solid hull-to-deck joint. Our Olson has a full overlap, thru-bolted joint, and the Ericson's are joined together inside with
roving. In either case this produces a virtually leak proof and really solid monocoque construction. When you look at the leaks (and sometimes worse) problems with the cheaper "coffee can" joints on the less expensive boats, there is a real difference - especially after 20 or 30 years..
Also, while other builders did copy the TAFG method/engineering, EY was a builder that really did it right, i.e. did not cheap out on the strength or scantlings. I do not know about the 70's interiors, but in the 80's EY was tabbing interior bulkheads to hull and deck; this adds a lot of strength. In our boat, virtually "everything is tabbed to everything else."
And then, all that structure stuff aside, Bruce King had a truly legendary "eye" for graceful looks that stand the test of time and transcend fashion changes.
(We do love the form-follows-function look of our Olson, but it kinda lacks the artistry of the BK vessels. )
Aside: just yesterday I had occasion to create a 1.3" hole in the little flat molded "step" right in front of an aft bulkhead where I am leading the cabling to the new shore power charger. Even this interior flat FRP area (2' X 8") is cored with balsa, like all of the interior flat surfaces. Better news, only the interior and the deck has coring; like most all Ericson's our hull is solid layup.
Having been inside almost
every part of our boat during the 10 month re-fit, we have been impressed with that care that went into its construction.
Admission: My opinions may certainly show some bias, but that bias has been earned and reinforced continuously since 1994.
Oh, and speaking of price, the broker that sold the boat to us also had sold it new, and in round numbers it sold for approx 90K, back in 1988 out the door. AFAIK the Ericson's of the era were even more expensive on a pound for pound basis.