Harvest Moon hasn't been a game to change its formula much. Farm, make money, expand, find a girl, farm some more. Winning deal for Marvelous, its developer. That changed a bit after the game went portable...well, MORE portable.
After Friends of Mineral Town, which was basically a port of the PS1 Harvest Moon game, and Harvest Moon DS, which was a SLOPPY port of the PS1 game (set in a different town), Marvelous foisted development of the PSP and DS HMs to different developers while they added some waggle to the Wii proceedings.
PSP harvest moon to the unknown ArtePiazza (porters of Dragon Quest V to the PS2 and concept artists for Dragon Quest III on the SNES). DS went to Neverland, the less below-the-radar studio responsible for Shining Force EXA and Neo, and Rengoku II (and a new Sakura Taisen game OMG).
The results were two games that were largely different from the conventional Harvest Moon, and dareisay, they're not entirely unwelcome.
Rune Factory is the second Harvest Moon game (first being the earlier-released Innocent Life on the PSP), on this gen of handhelds and the first to be placed in a fantasy setting. Harvest Moon has always been "fantasy" in the sense of having harvest elves and witches and things, but this is the first set against an orcs-and-elves, swords-and-sorcery Tolkien-esque background.
And that's the twist. Rune Factory adds swords and sorcery to our rustic retreat. And it works! In addition to your basic farming, you unlock and explore caves and dungeons and fight the monsters within. In the dungeons lie crop space that maintains the same seasonal weather year-round, so you don't have to wait for a given season to plant and harvest what you want. Also in the caves are ore patches that you can mine to upgrade tools, weapons, and farm expansions.
Also new to Rune Factory is an upgraded art style. Gone are the child-like sprites of ye olde, replaced with animoo character portraits that finally, FINALLY make it a little less disturbing to court the game's many available women. Sadly, the pickings, while many, are a bit shallow. There's not a large diversity of dialog, so it just feels like you're going through the motions when you try to hit on them or bribe them with gifts.
Also, characters in Rune Factory are represented by chunky 3D models on a well-designed 2D background. It works well enough, until you look at yourself. Then you look kind of like the monsters you kill in the caves. Is it social commentary ("WE are the monsters!
Speaking of monsters, they replace the old cows and chickens and are more useful for it. "Friending" monsters allows you to take them in and train them to water the plants, leaving you free to explore more dungeons or hit on more ladies. Call it Westminster Kennel Club, but with orcs instead of beagles.
Another twist on the old formula is an increased focus on stamina rather than time management. While energy was always in the harvest moon series, it was time you really wanted to keep track of. Now it's your stamina. You can't take a lot of items with you into the dungeons, and doing most things requires energy. It's easy to lose energy while spelunking. The key? Managing the in-dungeon farmland. Planting crops summons runes, which you can use to boost or replenish lost stamina, giving you more time in the field. Simple and effective.
Overall, Rune Factory gives some refreshing twists to the Harvest Moon formula, but in the end won't convert someone who isn't already liking it.
EDIT: Added a score for score-nazis.