The stylistic language is a huge one; probably the main one. I remember playing the original FFT back in 2002 and just plain not understanding huge chunks of the game because of mistranslations.
Well that's just darn cute. If you want that translation, Cheetah made FFT: Complete for PSX, you can just use that as a base so that's not a good excuse.
Mistranslated? What was mistranslated in the PSX version? No, seriously. Me and my wife looked at everything in Japanese vs PSX vs WotL, and WotL is the worst. Yes, there's "Two Swords" which doesn't perfectly describe the real use of the ability, but WotL didn't even TRY to translate some stuff properly at all. The Divine Sword skills: Shellbust Stab, Icewolf Bite and so on were translated perfectly on the PSX version (well as far as the limited character space allows), yet the WotL team decided: "Wow, fuck this. Just name them Rend Armor/Helm/Weapon/Accessory".
The only part that I'll accept that was mistranslated/sounded funny/weird were the spell quotes to some extent. What did WotL do with those? OUTRIGHT REMOVE THEM. LOL!
And you had trouble understanding what was said in the PSX version? Really? You're either stupid or you're lying. I'm not even a native English speaker and I never had trouble understanding the dialogue.
The polygons are also a lot smoother on the battle maps.
A personal choice, really. Native PSX resolution displays everything on the same level and it all blends very well. It's just less smooth if the camera is moving around. HOWEVER, since the PSP version uses a PSX emulator, you still have all the PSX jitter. So that doesn't exactly look very smooth either. So if you really care about this, you need to go with PGXP. That means using an emulator on a computer to play the PSX version.
Add on top of that extra storyline sequences (like Algus/Argath returning later in the game)
None of which are canon. This might be nitpicking, but Matsuno is a genius when it comes to writing stories. (Not when it comes to choosing a gaming company to develop his ideas though)
more abilities, 1.5 more classes (Onion Knight barely counts)
They use the same ability slots that the PSX version already had available.
The jobs are hardcoded like shit and you can't even remove the stupid 20 kills requirement for the dark knight.
The new items are also hardcoded all to hell.
Their sprites are quarter-assed (half-assed doesn't do them justice), and that includes all the other new sprites except maybe Balthier.
Funny thing, you haven't mentioned 16:9, which is the only advantage I'll accept for WotL over the PSX version along with cutscenes (and I really don't care about said cutscenes). The PSP just handles WotL 16:9 like shit though, everything is stretched and blurry like shit.
So, no, I'm sorry but I don't see how that makes WotL a good base for modding. At all.