Mild spoilers, read this if you've played the SNES version.
Neglecting the studying I should be doing I dropped a little over four hours into the GBA remake of FFVI today, just got Phantom, Bismark, et al. in the Magitek Research Facility and am about to flee via the train car Cid pushes you on.
I have to say, Celes, Cyan and Edgar's lines are a lot more satisfying, Ultros's lines seem slightly more perverted and there seems to be a greater amount of continuity between conversations. Also they kept the old American Character names (Sabin and Terra over Mash and Tina) however my two favorite characters seem to be watered down.
Locke loses all his great lines. "Call me a treasure hunter or I'll rip your lungs out!", being the one that stands out the most for me, is gone. I wish I had written down the discrepancies because they're rampant. The only other one I recall with certainty is when Terra casts magic in combat and Locke and Edgar stop to in awe. Locke used to say "This kid's loaded for bear." but now says "She's really something."
Next is Setzer who suffers from a milder version of Locke's loss of testicular mass most notable when he says that Celes is "cuter than Maria" and not "even more beautiful than Maria." However he does keep his signature introduction line "My life is a chip in your pile, ante up."
The Opera lyrics are different, and that jarred me. However I don't think they were worse, except for when they call Draco "a soldier of the West" rather than "The West's great hero."
Sabin is relatively equivalent. Except I did miss his "Confound it all! I'm starting to talk like you!" When he and Cyan were climbing into the M-Tek armors. Oh and they kept Magitek instead of going to Magitech, I liked that.
Other things are different, primarily item, move, monster and esper names but those are insignificant in any other reason than trying to recall which rages do what.
All in all, I think the step down in Locke's lines are greater than any of the benefits of a reinterpretation of the original Japanese.