In basic bits and pieces, the answer to your question is, the weight of the rails, no. of rail-anchors per rail, and the finish on the slates.
Although, there is an overlooked table that can be made to play "exactly like a Diamond." It's a "National Top Line," from the sixties. The sub-rails have to be extended, re- undercut, and cut to the proper pocket opening sizes. You also have to beef up the frame with modern structural beam material. You then you can change the rail-anchor number to 4 instead of three (more good news, these tables have the 4-screw round anchors, stock, so you only need 6 more anchors for this conversion) and the good news here is if you like the rails to play like a Diamond these come at 1 29/64" standard from the factory. Usually the slates are Italian from the 60's, so they're superior to a lot of slate today, although you do have to increase shelf size. And if the table has been in a home environment with moderate care the rails on these tables stay in good condition. There is one difference here in the rails, though, the sub-rail material is poplar (like the GC) and not Oak(like the Diamond) so there is a slight difference in over-all rail weight.
Like most everything else, the tables from the mid-sixties are the best. Usually, these tables can be bought "cheap" and once you've made the changes you and I have mentioned here, to the slates, rails, rubber, and pocket dimensions of the table, it does play with the same feel off the rails and banking characteristics as the Diamonds. The Italian slates that came on these tables have excellent surface speed (they were polished). Bad news is, you'll still have about $1800 initial cost and recon-work, not to mention the delivery, set-up, cloth, and new rubber, so all in, your still gonna have $3000. in the table. And the bad news is, "regular folks (non pro's) won't think it's playing as well as a Diamond just because of the name plate!"
I've done 5 of these over the years and the playability will fool even a great player in a blind test (no name plate or table identification)! One of these tables was right next to a PBT table and most players in that room liked the National, better. Now it's hard to believe that's a possibility, but certain tables can be made to play better than the pre-98 Diamonds! All in though, if you ever want to sell the Table, no matter what era the Diamond is, their re-sale value is excellent, like a Harley. However, if you really just want a table that can be made to play "GREAT," this is a little known, but really good way to go!