I haven't played many (read: any) modern RPGs, so I don't really know what types of changes would be jarring. In Diablo, do you have to tell each character what to do each round of combat? Because that's how Bard's Tale works. I don't know about the updated ones, but the originals didn't have automatic mapping and it was pretty easily to randomly bump into a party of baddies that would destroy your party. (The new save feature is worth it, for that alone.)
For me, the genre starts with Wizardry. It's a 3D dungeon crawler that exists within a 6 or 7 layer dungeon. When you leave the dungeon, the game just asks you where you want to go though a menu -- the guild, the weapon shop, and so on. Bard's Tale expanded on that by making you walk around the town and making you physically go to those places, and also adding entrances to multiple dungeons throughout the town. (That also means, you kind of need the map that came with the game. It mapped out the town, but not the dungeons.) BT2 expanded on that by adding 6 different cities and also wilderness encounters. BT3 expanded on THAT by introducing the ability to travel to different worlds. The coolest thing about BT3 is that it begins in Skara Brae, after it has been destroyed. It was mind-blowing if you played BT1 to death like I did; perhaps not so much today.
I've love for someone who's never played those games to go back and give them a shot, although I suspect the lack of modern features like auto-mapping and the slower based combat might turn off modern gamers.
I vote for #2.