Alright, let's get some tedious bookkeeping out of the way first. Persona 3 Portable (P3P) is actually the third iteration of Persona 3 -- the game originally came out for PS2, then there was a follow-up (subtitled "FES") on PS2 a couple of years later that made some tweaks and added some bonus materials, as well as an entirely new epilogue chapter to the game called "The Answer" that continued the story and was pretty long and involved. P3P contains pretty much everything added to FES in terms of little gameplay changes and bonuses, with the exception of the "The Answer" chapter. Also, in both PS2 versions, there were apparently animated cutscenes throughout the story, and like 95% of those were cut from P3P.
A bigger change is that P3P basically retrofits Persona 4's battle engine onto the game, and movement through the game world outside of dungeon/combat is handled in a menu-based "visual novel" style rather than actually walking a character through various screens and mashing X on stuff. I haven't played the previous iterations of P3, some people say the lack of animations and character models in general makes it less atmospheric, I can't really say either way. What I did appreciate was how zippy the pace of this game felt compared to the typical JRPG, and particularly compared to the frequent slogging of the first two Persona games.
The final major thing for P3P is the ability to now play the game as a female main character, which wasn't in either of the previous versions. While the progression of the game is pretty much the same, the story and the way you interact with other characters changes in a lot of ways, there's a shit ton of new dialogue and new Social Links (more on these later if you're new), and she even gets her own completely new soundtrack.
OK, now let's go back to the beginning for those who are completely new to the game. I hated on the first two Persona games for mostly the same reasons - sluggish pace, ridiculous amounts of grinding required, very sudden (and unfair) difficulty jags aplenty. The first game had a bad translation to English and a generally uninspiring (and somewhat confusing) story, but the second games (the sequel was split into two separate games) were much better written and actually had good characters and were engaging up to a point, but kicked you in the balls by getting sloppier in the gameplay and writing department as they wore on. On the whole both games often felt like they were deliberately trying to troll you quite often.
It's like Persona 3 was made specifically to address every complaint I had about the first two games. First off, one of the (numerous) things that cheesed me off about the previous games were that the game's dungeons were all locations within the city that you were stuck in, which were only open for a limited time window until you completed them, then they shut forever. So if you missed something in one and completed it, or just didn't grind enough to handle the brutal next dungeon, too bad for you jerk! P3 does away with that whole trollish structure, by making the game more like a Diablo-style/roguelike dungeon crawl. There's only one dungeon in the game, called Tartarus, which you just go deeper and deeper into as the game progresses. It's completely compartmentalized from the rest of the game in which you attend school, bop around town and socialize, etc.
Also out are the annoying "monster conversations" in battles, which just added another layer of grinding to the already-grindalicious previous games. You don't chat with any monsters here, you just annihalate them. You still go to the Velvet Room to fuse new Personas, but you get them by randomly drawing them from a card game that appears after battles, then you can fuse them directly together there to create new ones. You also get to register them as you gain them for free, then pay a fee later to re-summon them, so experimenting with fusions is more painless than it used to be.
Instead of grinding, you'll be passing more of your time in this one working on your "social links" with various other characters. This is handled pretty much like the typical Japanese time-management/dating-sim type game; you can only hang out with one or two people per day, and only on certain days, and you have to juggle raising people's affection for you with your busy schedule of monster-bashing (and the overall one-year time limit.) The character also has ratings in Knowledge, Courage and Charm that you can raise by doing various activities (which also eat up time.) Courage and Charm are mostly used as prerequisites for starting certain social links, but also unlock certain sub-quests for bonus items as well. Knowledge does both of those things, but as you're going to school you'll periodically have to take midterms and finals, the results of which are impacted by your Knowledge level. And to add yet another layer of complexity, there's a bunch of girls (or guys for the female MC) that you can date, but if you try to date more than one at a time, you'll make them mad and can "reverse" or even "break" their social link with you. You can also screw up social links by answering questions in conversation the wrong way sometimes, too. So it's like this whole secondary game on top of the dungeon-crawling and level-building. But it integrates with it, because stronger social links mean you can get an EXP boost to personas when you create them, making their starting levels and abilities more powerful. It's a handful, but a good handful.
The one thing that was maybe a questionable step from the prequels was the switch to a new composer for the soundtrack, who goes for a very J-Poppy style. Now, I'm no big fan of animu and even less of J-Pop, but I guess this is about as well-done as the genre can be, and some of it grows on you, even the Bieber-esque battle theme. Mostly thanks to goofy English-Japanese mishmash lyrics that have all sorts of endearingly hilarious mis-hearings waiting to happen.
On the whole I really liked this game though, and thought it was a major step in the right direction for the Persona series. The Tartarus dungeon and combat can maybe get a bit repetitive with a lot of palette-swap enemies and some of the music can get on your nerves, but otherwise this game is superbly executed in all phases. It's well-written and really well localized, has great characters and a really good overall story. And there's lots of little bonus things and reasons to replay it at least two or three times. Honestly I was giving it Medal consideration, but it's still probably going to be a little too JRPGish and Animu for the average Joe Budweiser to handle. If you enjoy JRPGs at all though, grab a copy while they're still being pressed and it's only like 20 bucks new or so.