Lets face it, Harvest Moon: DS was pants. Compared to the classics on the original Gameboy, the Gameboy Color and the GBA, the DS version was just a rehash, didn't make any use of the technology and was generally buggy.
Rune Factory is what the DS version of Harvest Moon should have been. Keeping the core elements of the series, Rune Factory (RF) adds in traditional Action RPG elements like swords, shields, dungeons and monsters. Players can play through without killing a single monster and can play through without planting a single crop (though fishing might help). It's made by the same people that produce the main series, so it's certainly true to form.
There is a fair bit of humour in this, and long standing fans of the series will enjoy references to old clichés. The player starts off as usual in charge of a farm in need of a serious touch up. The field has naturally been hit by some sort of hurricane and your house is barely big enough for the bed. Yet fear not, as through fishing, crop growing, monster slaying, mining and many other staple Harvest Moon (wait monster slaying?!) activities you can earn money and make the Harvest Goddess proud.
Crop growing is the safe option to build up funds, as is fishing once you acquire a rod to fish with. Tools can be upgraded so they use up less of your stamina and so they can obtain better items and perform generally better (for instance mining more precious materials, catching larger and rarer and more valuable fish, etc). Your house can also be upgraded to accomodate more items, a larger bed (wahey!) and other conveniences.
Now we come to the interesting bit, up until now, it sounded like a regular HM game. The new addition is monster slaying. There are a number of caves dotted around the world (dungeons basically). Each one has it's own microclimate and you can grow seasonal crops all year round in different caves. The caves also have monsters which you can go and bash about once you get a sword. The caves stand as a vague achievement system - they must be unlocked by performing various feats like "till 100 squares of land in cave X to unlock cave Y". Different caves have different monsters and also different things to mine/fish/harvest. Caves are levelled as well, and so you can plant health rejuvenating plants at lower levels to help you along. Phew, it's quite complicated but not over yet. One final thing is that there are no harvest sprites in this game - little gnome like fellows who help you out on your farm - you must now catch/befriend monsters to do it for you. There are some monsters which will do jobs for you like harvest crops, others will produce wool and milk type items for you to harvest, and so forth. You house them in little huts on your farm and you can have a fair number before you'll run out of space.
For veteran players there are a number of handy improvements. You can now walk over crops - negating the need for fiddly hoeing patterns - so you can harvest all 3x3 without extra help. The inventory is massive and everything can be sold to anyone in a shop. VERY handy for when you miss those shipments.
The interface is nice and clean, the graphics are good - although the character models (3D of course) are a bit low-poly - ie they look a bit low resolution and out of place. The backgrounds are often uniquely drawn rather than tiled, which is a nice touch - the town is particularly good in this respect. Control wise it works well, and the touch screen is used to good effect.
Overall, it's what HM:DS should have been. The RPG element is a good addition, and it's amazing how the developers have crammed so much into one game. The open-endedness of harvest moon continues and so does the fun!