For an upcoming campaign I'm considering making a house rule saying that A creature can take a move action that doesn't provoke attacks of opportunity to stand up from prone. Alternatively, I'm considering a house rule saying that A creature can also take a standard action that doesn't provoke attacks of opportunity to stand up from prone.
However, I'm concerned about long-term unintended consequences and game balance. What effect will either house rule have on the typical dnd-3.5e campaign in a setting that's similar enough to Greyhawk in which a party of two primary combatants and two primary casters—all between tiers 2 and 3—are expected to advance from levels 1 to 20?
Note: It was kind of the last straw. Polgolb, the level 3 goblin barbarian chieftain, was tripped by the level 2 bard's 150 gp trained-for-war riding dog. As Polgolb tried to stand up from prone, a PC made an attack of opportunity, rolled a critical hit (first a natural 20 then confirmed due to Polgolb's −4 penalty to AC), dealt Polgolb 20 or so points of damage, and sent Polgolb to his eternity with Maglubiyet. And, once again—for what seems like the thousandth time—, I was reminded of the incredibly asymmetrical nature of trip attempts and of how very, very few ways are available for a creature to defend itself from them and of how there is, so far as I'm aware, absolutely no way for the typical prone creature to stand up from prone without provoking attacks of opportunity.
For comparison, in Dungeons and Dragons, Third Edition, the action stand up from prone did not provoke attacks of opportunity. In fact, Sword and Fist says that "it is quite possible to remain on your guard while standing up" (68). But the 3.5 revision quietly changed this, and my bad guys have been falling all over themselves ever since. I know that to defend themselves against trip attempts creatures could devote precious feats like the Dragon #323 feat Stalwart (96) or the Planar Handbook feat Earth Heritage (38–9) or skill points to the skill Balance, the skill Tumble, and skill tricks (assuming such skill points are available) or money to items like from the Magic Item Compendium the boots of agile leaping (76) or from Underdark the armor accessory stability weights (66), but tripping still seems to unfairly favor the PCs: a PC that devotes even a smattering of resources to making trip attempts will trip all but the biggest or most well-prepared monster. This is made especially egregious as monsters—because of their limited feats, ability scores, treasure, or whatever—are quite often unable to use this the tactic effectively themselves!
Frame challenges are welcome, but bear in mind that as a DM I find devoting some of an opponent's resources to defending against some of the PCs' better known tactics reasonable, but that devoting enough resources to make trip attempts outright fail isn't this DM's idea of fun. Also, I don't view simply throwing against the PCs creatures that themselves make trip attempts as solving the problem so much as exacerbating it. Finally, while making gentlemen's agreement saying that everyone will avoid trip attempts in most circumstances is a solution, I'd prefer any solution be mechanical in nature instead.
Best Answer
Trip would go from cheap to worthless
The long-term issue would be diminishing the value of tripping. At low levels, PC's have precious few resources, so making trip less valuable means that it will see less play. That riding dog is a threat in its own right at 2nd level, but it cost some of the bard's gold. By even 6th level, the same dog is nearly useless; it's relatively fragile, and it's relatively weak.
Recommended Solution
I suggest mirroring how the Withdraw action changes merely leaving a threatened space. Thus it could be named Defensive Stand, or whatever, and be a full-round action, unless the creature is limited to a standard action (as with withdrawl, a nauseated creature couldn't perform this action) to stand without provoking Attacks of Opportunity.
This way any creature can take a move-action to stand, risking an Attack of Opportunity, but by taking a Full-Round action to stand, being properly cautious, they provoke no Attack of Opportunity. That way trip retains most of its value for wasting actions, and some creatures (low-no intelligence, or very aggressive ones) will still stand faster and provoke the attacks.