I'm curious if you have more about this elsewhere, and/or care to elaborate. I (effectively) grew up on 3.X, so I've never know anything but Full, Standard, Move, Swift, etc.Sure!
I think it has very much to do with how you play at the table.
The long and short of it is, it is massively powerful and easy to unbalance.
Currently, me and my friends, we play for four to six hours, once a week.
In that time we spend between 30 to 60 minutes in combat. They fight maybe five to eight different groups of opponents. Occasionally we'll have a much longer fight, up to thirty or forty minutes once every six weeks for a major fight, spending about 90 minutes of our four to six hour session in combat.
A fighter that cuts down an extra few opponents or a wizard with the occasional extra action from time stop makes very little difference. Your build, your skill at combat, how much you can accomplish in a round makes very little difference to your peers, because it's more about how you manage your resources across six to eight encounters.
Now, when I played Pathfinder, the reverse was the case. There was a four hour session with 20 minutes of setup and one to three fights, lasting the remainder of the time. This also was a great time. I don't like running games like this, but playing in them is plenty fun.
The problem with the action economy is that it is trivial to gain extra actions. Rod of quicken spell? Yes please! Animal companion? Don't mind if I do! Give up a single move action in addition to my standard to gain extra turns? That's tops! Familiars and mounts? Why not!
This isn't even getting into psion/swordmage/timestop/prestige class nonsense where characters can just straight out get extra standard actions every round.
In the basic core classes, you have ways to allow players to dominate table time of the primary action the table is expected to engage in. If you're spending four hours in combat, and my druid starts every combat by attacking and summoning, then I get three turns every round (Familiar, summon, and fighting spellcaster) - well, what the heck is that? If you build a game engine that expects and rewards game long fights, then allowing people extra turns; creates situations where there are explicit easy ways to gain an unreasonable advantage in combat.
An additional point, this is an issue with large single opponents. How do you build a balanced encounter with a single opponent when your four person party outputs 120 points of damage in a round? They only get one standard action, right? Or you're cheating to prep the opponent for the party. The character/monster equality was very harmful to the action economy in 3e in that way. I believe this problem is addressed in 4e, and anyone attacking a 1e dragon (He breaths, tail swipes, claws to his left, and buffets with his wings. . .) knows that it isn't a problem.
So I hope this addresses your question about why the action economy is less than a functional system.
Thanks for asking the question! I love to hear comments and read e-mails by readers. Feel free to write me at valis 'at symbol' oook 'dot' cz