Okay, based on what the reaction in IRC was, I think this is going to arouse some controversy. It would be good to get some debate going on, regardless, so let's hear your opinions on this.
Currently, the spell lasts for a duration of 1 round / level and dazes the target, if it's a PC. If it's a NPC, they turn from Hostile to Neutral (I think) and will stay that way unless you attack them or a NPC near them. Correct me if I'm wrong and the spell dazes NPCs too, I'm not sure if I've ever used it or seen it used on a NPC.
In any event, the effect that the spell should have on the target is very different, as anyone familiar with the PnP version surely is aware. For good measure, here's what the PHB states:
Player's Handbook This charm makes a humanoid of Medium-size or smaller regard you as his trusted friend and ally. If the creature is currently being threatened or attacked by you or your allies, however, he receives a +5 bonus on his saving throw.The spell also lasts for 1 hour / level in PnP.The spell does not enable you to control the charmed person as if he were an automaton, but he perceives your words and actions in the most favorable way. You can try to give the subject orders, but you must win an opposed Charisma check to convince him to do anything he wouldn't ordinarily do. (Retries not allowed.) A charmed person never obeys suicidal or obviously harmful orders, but he might believe you if you assured him that the only chance to save your life is for him to hold back an onrushing red dragon "just a few seconds". Any act by you or your apparent allies that threatens the charmed person breaks the spell. Note also that you must speak the person's language to communicate your commands, or else be good at pantomiming.
So here's my suggestion: Remove all game mechanical effects from Charm Person when it's used on a PC, save for the Will save. If it's impossible to differentiate between PC and NPC targets via scripting, it could be considered to either remove all such effects from the spell entirely, considering that it hardly ever sees use against NPCs anyways -- well, in my experience... -- or apply my suggested change to the spell and keep the daze, too. The addition would be a system message (that yellow text) explaining briefly the effects of the spell for the targeted character's player, if the save is failed. Make it painfully clear that it's only a reminder or a recommendation, not an enforced rule that has to be followed. The player would be free to ignore the recommendation altogether. It would be there only to
1) explain the (voluntary) effects for those not aware of the PnP description, and 2) encourage the use of the spell for RP purposes, reminding players that this is an opportunity for some fun roleplay.
If the change would be implemented so that the daze effect was removed, the duration of the spell could be adjusted to be closer to PnP, too. (Another system message telling the player when the effects end.)
I'm aware of some of the perceived problems involved. For one thing, the system message might seem like an order no matter how you phrase it, making the player feel uncomfortable if they don't play along. (Although I'll admit that that's part of the actual purpose, to a certain extent.) I also anticipate some people pointing out that it's kind of blurry just how far you'd be supposed to play along, considering that there are ways to harm yourself or others indirectly. To this, though, I'd answer that the player is free to play along only as far as they feel comfortable with. Besides, the same problem exists in PnP, doesn't it?
That leads to the third objection I anticipate, which is that if the target character's player refuses to play along, this could create some unnecessary bad blood and OOC strife between the players. However, the spell was never intended to give a leverage in PvP, save for a single surprise attack, so anyone using the spell on another PC with actual charming in mind would regardless contact the other player via a Tell and ask them if they would play along, so the problem would persist regardless if the target refused to comply.
Finally, I encountered in IRC the objection that no one should tell other players how to roleplay their characters. In my mind, this is a rather weak objection, since it's a fact that the spell Charm Person has a profound effect on how a character should be roleplayed. Therefore, given the leeway that I've emphasized of not having to comply, I don't think the suggestion would be a harmful one.
There, I'm done writing my essay. Opinions, please.