You run into two major problems with reviving a beloved but long-dormant game franchise. One is that the talent that made the original games great has usually either moved on from the company, or been promoted up the ranks so far they can't be expected to do the menial grunt work they did before that made the games great. The other problem is the changing whims and trends of market forces. The game is still primarily a for-profit product, and if the original gameplay is deemed too dated or out of fashion, it's looking at a pretty serious overhaul which may change the core experience too much from what the established fans like.
The Ys series took a long hiatus from 1995 to 2003, during which time some of the talent drain inevitably occured. But the games that have come out since then - Ys 6, Oath in Felghana, Ys Origins - actually struck a fairly good balance between a modernized polygonal look and the series' core action-RPG gameplay, without revamping the formula too much or cobbling in wholly foreign elements. All of that, for whatever reason, totally went out the window with 2010's Ys 7. Probably due purely to sales; VGChartz shows that none of the prior PSP entries did all that well, and sales outside Japan were almost nonexistent. The solution to sales problems that most of the PSP library attempted was to copy the hottest trends in the market at the time that WERE selling well - in this case, Monster Hunter and Korean MMORPGs. And so goes Ys 7.
At first blush Ys 7 doesn't seem *too* different from previous games. Red-haired swordsman Adol is still randomly wandering the world and somehow getting into world-saving struggles everywhere he goes, and doing so by flailing his sword a mile-a-minute at heaps of monsters. The first major difference you notice, however, is that you have a party now; longtime series comic relief character Dogi tags along now as a playable character, and you'll enlist even more mooks over the course of the adventure. Computer AI controls the ones you aren't currently, and they're perfectly reasonable about keeping themselves alive, but this ties directly in to the next major structural change - the platforming aspect of the game is entirely gone, as the jump button has been entirely removed. Aside from simply flailing your sword and tanking enemies, your only other recourse now is a quick-dash move.
Now, platforming in Ys 6 and Origins kind of sucked; I felt like it actually ruined both of those games in their latter stages. But I don't think the correct answer was excising it entirely. With it also goes the fundamental nature of Ys boss battles, which had a pattern-recognition style similar to that of Mega Man games. Bosses in this game still telegraph attacks that you use the quick-dodge to avoid, but battles now play out a lot more like Final Fantasy XII or a World of Warcraft boss battle. Your team spends a lot of time just standing and hacking away, and the main factor is no longer your reflexes or pattern-based skills, but whether you've level- and equipment-ground enough and brought enough healing items with you as you chip down the ridiculously huge life bar.
Compounding the new MMO-y style is a basic crafting system, which causes enemies to explode in a rather obnoxious shower of peices and parts whenever you kill them. For some reason, they also don't always give up all their parts immediately after dying, you have to beat the carcass several times to get the full complement. This is weird and I don't know why they felt it was necessary. You end up lugging around this giant inventory of Monster Balls and such that you end up using to forge new weapons/armor, and also for a number of your typical unimaginative MMORPG loot-grind fetch quests ("bring me 10 Goblin Peckers and I'll give you a tasty treat in return!")
Also different is that the game is about 2x the size of any of the previous Ys games. A lot of this time is padded out with heaps more dialogue and unskippable cut scenes; unfortunately, it's the same bland, cliche-filled (Powers of the Elements the central theme of the story for the 10000000th time), generic animu trope writing that the series is generally known for. This was always forgivable in previous games because it was kept to a relative minimum, with most playtime being spent in the fun action-oriented flow of the game listening to rockin' tunes while you carved monsters up. With more focus on plot and characters, the game feels like more of a slow-paced drag. Having to switch characters constantly to get the "right" attack for different monsters is also obnoxious and kind of pointless.
That's not to say Ys 7 is a mess; in fact, it's not even a bad game. It has some very nice-looking environments (by PSP standards anyway.) The soundtrack is often quite good (though still miles away from the Koshiro classics of the past.) The gameplay on the whole is very solid. I just think the core changes were made entirely from a marketing standpoint, and they're not changes for the better.