Turn-based games have a unique way of handling event timing. Instead of acting in real-time, turn-based games use actions to determine how many things a character can do in a given span of time. The action economy is something very difficult to work with, and any alterations to it can greatly skew the balance of a game.
The simplest type of action economy is single action rotations where each character gets one action per cycle of turns, and the characters go in order until everyone has taken one action, and then the cycle repeats. This basic concept is used by most turn-based games, although there might be some wiggle room with things like movement.
Anything that allows extra actions in this environment tends to be extremely powerful even if those things are very limited. In D&D 3.5 (and PF), there's a special type of action called a swift action that can be taken in addition to the standard or full-round action you normally get. These swift actions are often less significant than standard actions, but for the most part because they do something at all they are extremely powerful. Some swift actions are just as good as standard actions, but cost more resources (such as quickened spells).
Action denial also really impacts the action economy. Making an opponent lose an action by spending an action of your own is generally really powerful, but making an opponent lose more actions rapidly becomes ridiculous. Even two rounds of lost actions generally falls under the "save or die" category just because being two actions behind the curve means an otherwise balanced combat is now probably heavily skewed.
Clock-based turns are another way of handling the action economy. These systems have a number of problems, mostly due to initiative advantage rather than action economy. As an example, games like Final Fantasy Tactics or the semi clock-based PnP game Champions have faster characters acting first. Occasionally, faster characters also get more turns too if the encounter runs longer. Faster characters acting first AND getting more actions is generally bad, because their actions are, all things being equal, going to have the first chance of disabling an enemy. Even if the enemy isn't downed right away, they will win the battle in the long run simply by doing more things.
The only way a slower enemy can beat a stronger enemy in this type of economy is if their actions have more weight. Put simply, if they are more resilient/evasive than the faster opponent, they might shrug off enough of the opponent's power to get ahead. Likewise, if they are considerably stronger offensively they may be able to win as well. It really comes down to specific numbers, but going first AND having action advantage is generally very ridiculous.
The last type, and the type I like a lot, is some volume of actions that is measured during the turn order, with initiative a separate thing. Linking initiative with action economy is very bad for balancing because you have to adjust both to adjust one. The action volume is much nicer because you can make small tweaks or adjust it without doubling. Adding a single partial action to 4-6 base actions is a lot less impactful than adding the same sort of action to one. It makes balancing a lot easier because nudges aren't huge swings.
In the same vein are action point systems and I kind of view them the same way. If you have 6 action points and you gain 1, and it takes or 3 action points to do a full action, that extra 1 point might only mean a bit of extra movement or an extra small thing. Again, I really like that overall idea.
This is perhaps why I keep coming back to my homebrew Palladium games, because many of the issues with the system could be fixed with really aggressive balancing. Unfortunately you'd have to completely throw out most of the content (and the system has a lot of great content that was produced over the years) and make your own stuff, but I feel like the basic skeleton of the system is probably better than something like D&D where adding an extra action breaks everything.
Honestly I would publish my own homebrew Palladium content if they weren't so awful with their legal issues. The main issue with simply rebranding it as something else is really content. D&D has huge reams of material for their system. I used to have like, probably 40+ books for Palladium games worth of content. I'm good at rule systems and semi-OK at worldbuilding but producing enough specific things like gear, character classes, abilities, monsters/enemies and stuff like that is so daunting for me.
I dunno, I have time now so maybe I'll work more seriously at it.