Its been covered by
@Fictional Character but:
Firstly the PBTA ruleset isn't as bad as the minimal-effort offshoots would have you believe. Its not great, but its not complete shit either. What makes it complete shit is people just slapping "PBTA!" on a PDF that involves talking and rolling 2d6s.
Secondly, I will preface this by saying I personally fucking hate "Narrative" games, and I really hate the ones with shitty resolution mechanics, but I can respect that are people out there who like them and think that's fine. I will combo that by saying I absolutely loathe "follow these 12 steps to make a creative story!" creativity guides.
PBTA is an unholy combination of all three, and for a traditional game sesssion, sucks at both mechanics and narrative.
We'll start with Mechanics.
Look at the base resolution mechanic; Roll 2d6. 1-6, you fail. 7-9, you succeed "at a cost". 10-12, you succeed exactly as you say - its on the GM to tell you before you roll if what you're saying you do is impossible. On a personal note, I dislike systems that directly put the GM on a confrontational stance with the players with no basis to arbitrate against player arguments other than "cause I said so", especially when the subject matter is weird lands or people with extraordinary abilities, since you don't have "reality" to fall back on. Those debates usually just turns into unfun arguments and players feeling shutdown by GM fiat if they don't get their way, or feel like the GM is being spiteful when something goes wrong for them later on. Maybe you have a group of people who just say "Oh, OK, GM said so, that's fair" - in which case I want their names and contact info so I can verify, and also steal your elfgame friends.
Also I dislike the "happens as a you say" resolution- If your bard walks up to the King and you use your diplomacy to tell him you want him to suck your dick, rolling a 20 does not get you royal blowie, it gets you the King not immediately ordering your arrest and execution.
Lets look at a 2d6 probability table (formatting is probably going to get fucked up and I'm too lazy to tablize properly so it won't get fucked up;just google one if you really care and want to follow along)
# = =< >=
2 2.77 2.77 100
3 5.55 8.33 97.22
4 8.33 16.66 91.66
5 11.11 27.77 83.33
6 13.88 41.66 72.22
7 16.66 58.33 58.33
8 13.88 72.22 41.66
9 11.11 83.33 27.77
10 8.33 91.66 16.66
11 5.55 97.22 8.33
12 2.77 100.00 2.77
So from the table, you have about a 17% chance of your action doing off without a hitch (For d20 purists, roughly rolling 18 or better). You've got a 58% (~9 or better) of your action succeeding in general. Only a 42% of complete failure (~8 or below) and the GM gets to fuck with you.
You also usually have Stats of some kind offering a bonus/penalty usually -1 to +2, and often some abilities will give you a +1 for a future roll. That means if you are using something you've got a strong stat in you're looking at 83% odds of success (~3 or better ) and if you line up a +1 bonus from an ability, you're looking at 92% odds of success (~2 or better). If you can scupper an extra +1 from somewhere, like relationships, you're at 97% odds of success, so even better than "Don't roll a one" in d20 land.
On the other side, if you have a -2, your odds of unmitigated success are 3% (So worse than needing to roll a 20) and complete failure rises to 72%. (~14 or better)
Most of the PBTA spawn also include an advantage/disadvantage mechanic, and I'm not even going to bother with the probability there.
This is a long winded autistic way to say that PBTA, like a lot of bell-curve systems, when things go bad they go bad in a big way.
Unlike other 2d6 systems I've seen, PBTA-spawn lean heavily on using pluses and minuses, and constantly adding or removing them. Its got probability scale issues and usually no warning to GMs about how much more powerful a +2 vs. a +1 is (or how much more harmful a -2 vs -1 is)
Because PBTA is primarily a narrative game, it uses a lot of open-ended descriptions for when things apply, including the 'healing' mechanic. In PBTA-spawn, you usually don't have hitpoints, you take damage to your abilities as negative status. You take all the damage check boxes, and you are defeated. So you can relieve your character of these status effects by performing narrative actions; i.e. Remove Fearful condition by being brave in the face of danger.
Again, no real distinction on "Brave" or "Danger" and limited guidelines. This is ripe for arguments and slap fights, and the arguments come down to the GM deciding if they felt that was brave or not.
Its a horrible implementation of Narrative interacting with the Mechanics.
On that note, now lets go over how they fuck with the narrative side of the house.
PBTA-spawn is driven by "moves", both players and GM. There are usually base moves, "Talk to someone" "fight someone" "interact with something" - and then moves based on class. Some of the moves have a "pick from this list" as a result - i.e. if you have a "Punch a dude in th dick move" you might have on 10+ select two from the following list. 7-9 pick one.
- Pisses blood for next week
- Voice becomes high pitched until
- Gets a boner
- Takes -1 to all penis-related moves
You're limited by the choices, some of them leave a lot to interpretation, and not all to the choices are equal, some are clearly better than others. It'd better to just give the Move the best options and say "you can also swap out for this".
On the GM side, the GM "moves" are basically just creative writing prompts "Take their resources", "reveal an uncomfortable truth", etc. Its like one of those books that purports to teach a structured approach to creative writing.
PBTA-spawn don't have turns. Its based heavily on "narrative interuption"; i.e. the GM says what's happening and the players jump in to change the story. Players go when they shout they do a thing, so if you've got shy players (or a particularly enthusiastic player) you can have people getting shut out because they aren't quick enough on the draw.
tl;dr You're getting the worst of both unbalanced mechanics and poorly thought through narrative.
I'll give PBTA some damning praise. I appreciate the way it encourages (read:forces) player character interaction and places a lot of mechanical emphasis on character (and usually NPC) interaction. Narrative games are not my jam, but I appreciate the way the docs (at least of the main line releases) directly encourage the GM to engage with players and encourage them to drive the action. I do like their "Damage manifests as increasing penalties" system, but feel it not properly balanced and leads too quickly to failure cascades.
In short, if you don't care about the fucky mechanics and just want to do a story telling game with your group that involves rolling dice, there is a PBTA-spawn called 'Simple World' which is the closest thing I've seen to a "SRD" for PBTA, and you can just roll your own Lesbian Warrior adventure without needing to give money to some soy-faced feminist ally for his fetish material.
If you want combat to come down to a "and here is how I killed all the monsters", PBTA excels here (and to be clear, I'm not being facetious. Sometimes you want combat with a little crunch ,to exist but for it to be quick and secondary to the narrative).
If you want a traditional hack-cast-slash, PBTA is terrible. And if you try to bring up Dungeon World I
will judge you.
I'll now turn it over to
@Adamska to re-say most of what I just typed, but competently.