Let's talk about a flashpoint that was released relatively late into the game and yet still managed to have a huge effect on The Old Republic - both by introducing a successful new concept to the game and by showing how not to do certain things: I'm talking about Kuat Drive Yards.
Kuat Drive Yards, or KDY as people quickly started to call it, was added as part of patch 2.6, Galactic Starfighter's official launch, in February 2014. It was the first true tactical flashpoint in the sense that it was both role-neutral and allowed characters of different levels to play together. The term "tactical" was specifically introduced to describe this, though it was mostly applied to flashpoints that were simply role-neutral in the immediate months afterwards, since there were no others that had level scaling.
To this day it only has a single difficulty, without a solo or hard mode, though at launch it was at least split into a levelling and a max-level version, which is no longer the case.
KDY was also the first and only flashpoint to attempt a sort of "modular" design, in that each run puts you up against two out of five different scenarios and one out of three different end bosses, making no two runs exactly the same.
Your enemies are combatants of the enemy faction, though they are extremely generic. Having been designed to be defeatable by a group of level fifteens with no useful skills, the vast majority of mob groups in KDY are extremely weak and barely any larger or stronger than something that you might encounter while questing solo out in the world. The most likely thing to challenge your group if you don't have a healer are the random mini bosses in each scenario, as they have champion levels of health but are not surrounded by kolto stations, so your group needs to find a way to chew through those hitpoints without heals if necessary.
As mentioned, you are up against a random two out of five possible scenarios in each run. The options are:
Armoury: Run around blowing up some weapon stockpiles.
Cannons: Seize some cannon emplacements - in practice, this doesn't feel very different from the armoury; there are just fewer things to click on and the room has a different shape.
Hangar Bay: This one has you fighting off waves of mobs that are trying to take over the hangar, which makes for at least a bit of a change of pace. Also, the last mob spawn is actually at least moderately hard and can cause people to die.
Prison Break: Free a bunch of fat twi'leks and thin cathars in orange suits from their cells. They are really fragile and die easily if you run into combat while they are following you. This doesn't cause any issues with completing the scenario, however there are achievements for getting them all out alive... and for being the one person to open X number of cells, which initially led to friction among pugs. (My impression was that things calmed down considerably after the initial rush though.)
Starship Assembly: This one is interesting because it has a puzzle component, as you're supposed to create a working starship prototype and need to decipher a minor logic puzzle from some nearby datapads to figure out which parts are the correct ones (there is more than one version of this as well), else it blows up. Picking the wrong parts won't affect your overall completion, but picking the right ones counts as completing a bonus mission, which is neat. This can be very hard to get right in a pug though, as people will just click random consoles and cause the prototype to be assembled incorrectly.
The three end bosses of which you face one at the end are:
Master Khoris/Lord Modo: A Force user that jumps around like a bunny on crack, which makes him very annoying to fight.
Major Benes/Lieutenant Krupp: A soldier that throws grenades at everyone that the group needs to cleanse at a nearby degaussing station. I actually really like this mechanic because it's forgiving enough to be pug-friendly (you don't immediately wipe if someone messes up) but still demanding enough to require your party to achieve some form of co-operation (if nobody bothers with the grenades at all, people will die).
Station Guardian One: A not very exciting droid that summons some adds.
One thing I've bemoaned about the fact that Bioware made all the old flashpoints tactical in 4.0 is that this made a mess of their stories for new players, as there are no clear in-game indicators of what order you should play them in.
Kuat is interesting in that regard because it's the one flashpoint that was actually designed to have a level-neutral story from the get-go. The intro quest is even slightly different depending on where you are in the levelling curve when you pick it up. At the same time, the story itself is so far removed from the main storyline that it doesn't really matter when you do it. It's just the Republic and Empire fighting over a shipyard basically, which is the kind of thing they would do at pretty much any point in time (even post-KotFE).
There's also a tie-in with Galactic Starfighter, as you meet the NPCs that serve as default co-pilots near the flashpoint entrance and there is a comment or two that you can also "help the cause" by jumping into a starfighter and engaging in some space pew pew.
Kuat Drive Yards was a huge success on release. Letting people play together regardless of level and role meant that queue pops were incredibly fast, and people loved it. Levelling purely through flashpoints became viable for the first time.
There was a downside to this though: particularly during double XP events, people would chain-run Kuat to quickly level alts past any semblance of sanity. I saw it with some of my own guildies: They came to loathe the instance but didn't want to stop because the XP was so good. I'm convinced that this was a major factor in Bioware coming up with the first class story levelling event, because they saw that people were playing in really stupid ways just to gain experience more quickly and needed to be directed towards more fun and varied content to not burn themselves out.
At the same time, the role- and level-neutral flashpoint concept was successful enough that Bioware converted all flashpoints to it in 4.0. They also evolved loot scaling so that people can actually get level-appropriate gear from tacticals now. Kuat initially didn't have this, which is why it drops no loot other than reputation tokens. In a post-KotFE world, this seems oddly outdated. At the same time, Kuat has remained the only flashpoint that embraces the tactical concept from a lore standpoint and has a story that makes sense at any level.
The modular design doesn't seem to have been that much of a hit, as it was never repeated in quite the same way, and even KotFE's Star Fortresses only made cautious steps into the same direction again, presumably because ticking off randomised objectives didn't really fit in with Bioware's focus on story.