I have been kind of "going to town" on this game and really ripping into it for its many flaws and how it appears to be a game that could have been great if they had just finished making it (but to be fair after 10 years it is probably time to go ahead and admit that maybe it isn't possible to finish it.)
There is one major problem to this game that kind of overrides all the other aspects of the game; good or bad. This is the extremely broken combat system that while I think was well-intended when they were conceiving it, simply didn't work in the end and this is the main reason that this game simply can't be good. I will do my best to be impartial and just give the facts rather than appear to be a winge-boy while writing this.
No curing class at all
Part of the fun in the past was having a character on your team that was your dedicated healer. It was always very important to keep this character alive by all means. You would put them on the back lines and their primary purpose was support by way of healing, buffing, and staying out of the fray as much as possible.
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Everyone can heal at any time unless they have been turned into a frog or are petrified. This also doesn't really seem to interfere with their attacking progression.
Noctis (main char)'s own attacks hurt him
Noctis is the only character in the group that can achieve "warp speed" and dash from great distance to strike an enemy for additional damage. However, it is extremely annoying that doing so also damages you. I mean sure, it is conceivable that flying across the room at light speed would damage you but Final Fantasy (and all RPS's) aren't exactly about realism.
I don't know how many times I actually depleted my life bar because I foolishly continued to attack because there is so much going on in any battle that you don't exactly keep an eye on your health bar. Your main attack as Noctis is doing warp-strikes, yet this can actually kill you. WTF?
You can roll-dodge almost anything
If you are having a really tough battle you can simply spam the square button and avoid 95% of all attacks in the game. This is even possible if you are currently sitting under a massive dragon that is trying to stomp you into oblivion.
It doesn't really matter what is attacking you, the dodge-roll evades almost all damage. I have only found one kind of attack that it definitely wont dodge, and that is when this one particular samurai enemy does a "katana sweep" of the entire area in front of it. That'll get ya!
Using items negates all activity around the toon
Every now and then you will be facing an enemy that has a super powerful attack and you can see that it is coming based on certain things it does to build up to this. In one instance you can see a dragon powering up its electric strike (sorry, but i don't recall the name of said drago) the moment before he strikes, the idea is that you are supposed to get behind a wall for cover but if you aren't really feeling up to that you have another out.
Using a potion (or any of your other items) right when this strike is about to occur will start an animation process that doesn't stop the passage of time for the rest of the action, but it renders you completely immune to all damage that is in the process of about to happen. Since the animation actually takes quite a while (relatively) it is EXTREMELY EASY to completely eliminate these massive attacks from boss characters. Not what it would really matter anyway because of my next point.
You will never lose any battle if you bring enough potions / phoenix downs
Especially for potions these items are extremely cheap and accessible everywhere. If you bring something like 20 phoenix downs (these resurrect you, in case you are not a FF player) and 99 potions, it is entirely possible to win every battle in the game without having any strategy whatsoever. I didn't try this on EVERY battle but I honestly believe that if you brought these two items with you, you could simply hold down the circle (main attack) button and not even touch the other buttons for any battle in the game and while it would take a really long time, you would still win.
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By holding circle the toon you control (almost always Noctis) will automatically attack the nearest enemy and your partners do this automatically. Then whenever someone gets low on health you use a potion on them and like I just mentioned, this makes them invulnerable for a time.... You can't lose
I played through the entire game, as well as something like 50% of the optional sidequests and only ever died ONCE. This was very early on in the game and I had no idea what was going on, if I had known what was going on, i wouldn't have died that time.
You have loads of techniques but will end up only using one of them
The techniques part of the game are special abilities that members of your crew have. These trigger an animation process that is actually pretty cool and results (normally) in a team effort to do bonus damage to (normally) a single opponent. The "skill tree" that you must progress using points gradually gets you to a skill called "Overwhelm" (it isn't even a top-level technique) and once you have that one, it is so effective in virtually any situation that there is no reason to use any of the other techniques ever again.
If you have one particular weapons equipped for Noctis called the "Bow of the Clever" the damage you do it just off the charts. There are very few enemies in the game that stand any sort of chance of surviving this. I don't know if this is an easter egg or a bug, but once you have this technique, you will never use the other ones ever again.
Final boss is mindless
Oh the ol' final boss battle. It is meant to be epic, right? Well after all that running around and finally facing your nemesis in the battle to end all battles (intentionally not saying what it is to avoid spoilers) your team-mates are taken away from you and it is a one on one battle. You release all of your rage and subsequent powers and take to the sky to use your super-mighty-I-had-this-ability-the-entire-time-but-never-used-it skills and then simply hit one button over and over again occasionally using a potion to ensure you never actually get even close to losing.
tl;dr
I know that went on for a long time and my apologies. When you are passionate about something it is really easy to go on and on about it.
The combat system in FF15 is different than anything they have ever done in the past. The completely eliminated the turn-based combat and while I think this idea was probably a good one in committee, it's execution in the end is terribly terribly flawed.
I think that over the 10 years they were creating this system they probably realized about 7 years in that there was nothing they could do to make this non-turn-based combat system work the way they had intended, but at that point they had already dropped a tremendous amount of time and money into what they already had. Therefore, I think that Square-Enix pushed out a combat system that they knew wasn't going to work the way that had intended.
The biggest flaw is the "have potion / will win!" system. This rather insurmountable advantage on the part of your squad makes it so that there is no situation you will encounter that you can't win. You might get frustrated about how long it is taking and elect to abandon the fight (this has happened to me many times when I took on an enemy 20 levels above me or so) but if you simply stay and endure the tedium of constantly using potions, you will eventually win.
If the combat in a game that is centered around combat doesn't work, the rest of the game can't possibly work. This is true regardless of the fact that this game is absolutely beautiful, has a wonderful soundtrack, and has enough content to keep pretty much anyone engaged.
You don't need a strategy in this game in order to win. There is no incentive to ever improve your skills or ever progress beyond the main strategy being that you have to remember to reload on potions before going to the next big battle. Combat is lovely and wonderfully animated, but none of that matters when the overall system is so terribly terribly flawed.
I think maybe they bit off more than they could chew in the design phase and released a product that they were very aware didn't function the way they originally intended. Shame!