The subtleties of conversation are difficult to translate to a mere video game.

I like the conversation of Black Isle games, especially if the dialog is written well. Planescape: Torment is a prime example. Naturally, I figured they could just emulate this type of system, like Fallout 3/NV did, Mass Effect, The Witcher, etc. The problem is the way TES skill system works. You learn by doing. In those other games, you could advance your conversational ability simply by selecting that skill to advance when you level up. You don't do this in a TES game, and it doesn't make sense to repeatedly select a dialog option you know is wrong merely for the sake of practice.

I can see what they were going for in Oblivion. You try to read facial expressions to find out what the character will respond to best. It translated into an awful persuasion system, but I feel their pain. This is not a simple problem.

Perhaps a bit of Daggerfall could make it's way in here again. Daggerfall had a list of topics, similar to Morrowind, but you could select whether to speak plainly, roughly, or formally. When addressing nobles, it's best to speak formally, but the rabble on the street responds better to roughly.

Picture this. You select individual dialogs, similar to Fallout or any Black Isle game. But there is a small menu above the dialog options to allow you to set the tone of your response. Each response would change depending on what you select.

Formal:
Greetings my Lady. I am but a weary traveler, and if you would be willing to inform me as to the location of an inn, I would be most appreciative.

Plain:
Good day. You wouldn't happen to know where an inn is, would you?

Rough:
Hey wench. Tell me where the inn is.

Different characters would have different tolerances for improper behavior for their own status. The game would check each attempt at conversation against your persuasion skill to adjust the disposition of the character you are speaking to, so you are, in a way, always in "persuasion mode" if this were Oblivion, but subtle, and you aren't able to make someone go from hating you to loving you with just a few jokes. It takes time to gain someone's trust.

The higher your persuasion skill, the more you are able to influence their disposition in a positive way, and the higher their disposition toward you, the more they are willing to overlook anything you might say wrong. And very high persuasion skills could unlock previously unavailable conversation topics.

So you always have to watch what you say, always, lest you offend someone. And simply speaking to people gives you experience toward persuasion. Repeating things said before with the same person won't influence your persuasion anymore.