DDO would've been more or less exactly like the game we know and love today, but maybe with a much more "accurate" challenge-scaling system instead of per-quest difficulty options, and a few other tweaks.
Now, the Hardcore
setting, just like its namesake in BioWare's Infinity Engine games (Baldur's Gate etc. or Neverwinter Nights,) could have implemented a much more strict compliance with the "pen and paper" D&D rules. Such as,
- Attack bonuses decreasing instead of increasing with every attack in a sequence.
- "Appropriate" penalties for dying.
- Multi-classing penalties. *wince*
- Heavy currency (i.e. not weightless.)
- Lore Checks for Identifying items before their specialities can become usable. (not sure if this was in P&P, but definitely in all the D&D CRPGs AFAIK.)
- Maybe no DDO-specific alternatives for unimplemented P&P features (such as Prestige Classes vs. Enhancements)
among lots of other differences obviously, way too numerous to all list out (although you can see for yourself at DDOWiki.com's page
about the same.)