Saga Frontier is one of the most oddball rpgs to have been released on the ps1, and the ps1 was long considered the 'era of classics' for all jrpg-dom, with remakes of the earlier major names and innumerable newer classics being released for the system. The Saga series in general is something of an acquired taste, due to the sheer opaqueness of the leveling mechanics and odd, often nonsensical experimentation with random game elements.
In terms of game mechanics, Saga Frontier is easily the most well-explored of the series. It's devoted cult of fanboys and -girls have revealed everything of consequence about the original version over the years, so how much is this changed from the original?
First of all, for those of you who have played the original, a number of quality of life improvements. The ability to quicksave and autosaves at important points mean you are far less likely to trap yourself into having to spend hours regaining progress. The immense expansion of save slots makes it possible to backtrack when you underleveled for the final zones. As important in its own way, the spark trees and probabilities for weapon/fist techniques have been unified into a single one usable by all human and half-mystic characters. Last of all, New Game+ lets you bring over character progress, money, skills, and items between paths, making it unnecessary to keep re-leveling them each time you start a new character path.
However, for true fanboys, the true wow moment is the added content. Now, Saga Frontier is a game that is very sparse on active storytelling. It has the same 'silent presentation of the environment' that can be seen in a lot of the pre-2000 jrpgs taken to extremes, and this is the primarily element that takes the Saga series into a niche of its own. More than anything, I loved that they added in all the cut-out content from the original... especially a ton of content in Asellus's story, which was my favorite and most painful (leveling Asellus is painful at first) path from the original. This includes extra ways to get out of the first area (depending on your method, you end up in different places and have slightly different experiences), the fixing of the old Asellus path bug that sometimes made it difficult to get the Half-mystic or human endings, and a general polishing of the experience in general.
The other chunk of added content is the Fuse path. In the original game, Fuse was a curiosity of a character usually gained too late for him to be of any use (the method for obtaining him requires confronting a firebird on steroids who can incinerate your party in one hit). In this game, he has his own path, which is essentially his case files on each of the characters where his own work intersected with theirs. Since these are always told from his - slightly narcissistic - point of view, they are highly amusing to someone who has finished all the other paths. While this path doesn't add anything story-wise, it is still funny to go through.
Now for my assessment of the base game...
First, the most opaque of the game mechanics, the Battle Rank System. To be honest, this is perhaps the easiest way for someone new to the series to self-sabotage. Every battle you undertake, whether it is against a slime or a dragon, upgrades your Battle Rank (which is unseen), eventually unleashing upon you a new tier of enemies for you to fight. To be blunt, if you sit around in the first area killing the same things, the game punishes you with monsters that can one-shot you in the next area, because you raised the battle rank too high. This means that grinding essentially requires you to leave whatever dungeon you are in every once in a while to upgrade the enemies into something you can actually gain upgrades off of.
That brings me to the second unusual mechanic, the leveling system. Saga Frontier and the series in general doesn't have linear numbered leveling such as is seen in Dragon Quest or Final Fantasy. Instead, depending on probability mechanics and what you did in the battle (hit stuff with the sword alone, used techniques, used magic, etc) you gain a stat upgrade of some sort at the end of battle, though this slows if you are fighting enemies below your character's (hidden) level. What adds an element of annoyance to this is that the probability of whether you get any gains or not is heavily dependent on the enemy group you encounter. This makes only a few of the scaling areas attractive for grinding (pretty much just the Bio Research Lab, which always has the top tier of the battle rank).
Last of all is sparking... which is the process of gaining new skills, techniques, and magic. If you use a sword in combat, your character will not only gain physical stats at the end of battle, but there is also a chance - based on the enemy's 'spark value' (a hidden factor) that your character will be inspired and obtain a new tech or magic. Sword and fist techniques are sparked randomly based on which tech or whether you simply used the weapon devoid of techs. Gun techniques are based off of you using a gun and are given to you after battle. Sparking magic requires that the character using the magic have its 'gift' (gained through quests), have used that school of magic during the battle, and the character's intelligence stat.
For story, Saga Frontier will always be a somewhat disappointing game, despite the sheer beauty of its visuals. There will be innumerable times any given player will have wished for a few extra lines to flesh out a dialogue or for clearer hints to find the next place you need to go. The fact that NPC dialogue doesn't really change significantly between paths says a lot in and of itself, and this is a common complaint for people who play any Saga game.
The fact that I found this to be an immensely enjoyable game is as much nostalgia as the game's actual worth.