mqstout: I guess the one I almost came to liking was Bard's Tail Construction Set, but that's because of tinkering. Not because of the game itself.
It's "Tale", not "Tail".
I remember that game having a spell editor, except that it was too limited, and couldn't even replicate spells that were simply combinations of two other spells. You couldn't replicate Flesh Restore IIRC.
Forgotten Realms Unlimited Adventures was another game that would allow such tinkering, as was Blades of Exile (though I'm not sure about the release date of that shareware game).
mqstout: I also liked that drinking a potion left you with an empty vial, that you could then refill from a few of the potion fountains through the world.
I remember Dungeon Master doing this, except that refilling the flask (or simple filling it in the first place) simply required casting a spell. The game did not have any healing spells, but it did have a spell that would convert an empty flask into a healing potion that you could then drink.
(The game also gave you the option of filling the flask with water at a water fountain, in case the waterskins you find weren't enough.)
Cavalary: Not counting M&M6 since that's also a '98 game, like BG.
Didn't realize that game came out so late.
Some other features I personally like:
* Being able to recharge items with a spell (Might and Magic series, Wizarrdy 7, Dragon Wars)
* Being able to enchant items (Might and Magic series has the beginnings of it, but it's in games like Demon's Winter and Daggerfall where this mechanic is developed further).
* Custom spells, like in TES: Arena. (If that counts as an RPG; it's certainly more RPG-ish than Oblivion.)
* Dungeon Master's magic system, where if you know the runes for a spell, you can try to cast it. (The JRPG Rudra no Hihou ends up having a similar feel; if you know the name of a spell, you can cast it, MP permitting.)
* Bard's Tale's persistent summons. Summons aren't just for a battle or a limited time.
* Centauri Alliance's transformation spells. You can transform into a goblin or dragon, and it persists until dispelled or LIFE (HP equivalent) reaches 0, though being transformed prevents you from leveling up (but not from earning XP).