They made some gameplay updates, but they were basically just placed on the existing game. So, for example, you can do a FPS aimming thing, but the AI or level design was basically the same and it made things too easy.
All the real changes were done to the cutscenes and voice acting. Many of the cutscenes were changed to be more over-the-top and it doesn't work. Metal Gear has this sort of grounding in it's action that's not realistic, but it's got more artistry to it. This was just like taking a cheap hollywood movie or bad anime and putting it over some stylized and interesting. It worked on maybe one thing, but even then it was pushing it. Then they also revoiced the entire game, which is lame. If anything has aged well with MGS it was the voice acting. So when they redid everything they did stuff like dropped accents on characters or just did a little things so the inflection was different.
At the same time, they had the oppertunity to update or twist things in terms of boss fights that would have really been cool for the fans, but they left them alone.
I dunno, the whole thing felt like wasted opportunity... sorta like the Special Editions of Star Wars. While cleaning up source would be welcome, changing things that didn't need to be changed are not. I know I'm making a big deal out what really ammounts to several small things, but I feel that the things they did change were so close to perfect in the original game that they really went after the wrong elements.