The issues people are complaining about the most: UI, AH, lag etc are also the easiest to fix. You see, while these issues are annoying they are also known to be fixable and many people will just keep paying their monthly fee waiting for the fix because, really, there is no reason for them not to be fixed. So why they are not fixed now you ask? I have a theory.
Fixing these will draw attention to much deeper problems that are not easy to fix. In fact I don't know how to fix them without losing even more players.
Let's take a look at the Armoury system for once. Basically you can equip almost any skill on any class. Without restrictions this would be impossible to balance, everyone would be a tank that can also nuke, deal heavy melee damage, buff, enfeeble and cure. Imagine in FF XI you could utilize this system? Want to level THF - no problem, equip Madrigal from your LV75 BRD, Regen from your WHM, Utsusemi from NIN for a good measure and proceed soloing ITs from lv1 to lv50 (by that time you will be too bored to continue).
So, naturally, FF XIV has to put some restrictions. A good solution is to award SPs only for using your class "native" skills. This seems fair - it encourages people to use the class they are leveling. However this also means it's now impossible to get a predetermined amount of SPs from a mob similar to the XPs, this means you need to get "skillups" because there is no way to know how much the native skills contributed to every monster's death. Skillups are not bad by themselves, many games, including Square's own had them without infuriating players. However skillup system is easily exploited.
If you give a guaranteed portion of SP per action then people will hit highest level mobs, get crazy SPs, die, zombie, hit again and so on. So you need to hold the SPs off till the mob is dead but then it means you can accumulate crazy amount of SP by hitting a relatively high level on a low level character because you won't be leveling up and your hundredth hit will give as much SPs as the first even though you have already acquired enough SPs to be higher level than than the mob you are fighting and your SP per hit should be much lower (for example: lv1 hitting lv10 marmot gets 200 SP per hit, in a game like FFT you'd hit it three times, got 600 SP and leveled, getting, say, 150 SP for being a lv2 hititng a lv 10 and after five more hits you'd be a lv3, getting only 120 and all this while hitting harder and harder so the mob is probably long dead).
To prevent this you'd have to cap the SP from a mob but then people would reach the cap with their class native attack and proceed to nuke it with the equipped "alien" abilities because there are no skillups left anyways.
So there is no cap but there is also no guaranteed skillup, the system that we see now.
It works nicely for its purpose however it does seem to annoy players, forcing them to use lightest attack in hopes to get as many skillups as possible. Not to mention that there are about 10000 other MMOs that don't force players to grind RANDOM skillups. Some people like gambling and might enjoy this but it seems to me that they are a very small minority. At this moment very few people think about how bad is this system (skillups, not the Armoury itself) because they are distracted with other bugs. But are you really going to enjoy grinding these skillups when AH will be in the game? Unlike AH there is not much that can be done about it as I described above so it's not going to be "fixed". Of course, scraping this system and doing something that worked in other games (and I don't mean WoW, I mean other Square's games) could have solved this issue but Square seems so much invested into the Armoury that I am very skeptical.
And Armoury is just one thing that seems to be most obviously broken, I've only played for few days and did not see most of the game, if their flagship Armoury is as badly designed I cannot imagine what else is broken in a similar way. It appears that somebody at Square has realized this as well and now they are facing a hard decision. Fixing UI lag won't make or break this game at the moment. They need to know how to fix fundamentals or else they could as well shut down the servers right now.