I played Harvest Moon Back to Nature on my PS1 when I was young and really loved it! Then, I bought Harvest Moon Hero of Leaf Valley soon after it came out and I never regretted buying it. Unlike the first Back to Nature game, graphics for character are much more close-up and sharp (I play it on PSP). Plus, the scenery is more beautiful if you want to enjoy nature. The farming plot is limited, of course. But I think it isn't a problem as you can walk among your plants (again unlike Back to nature) which gives you more space. Many improvements!
And I'm not sure but there was no sprinkler in Back to Nature, right? Leaf Valley offers you a water sprinkler!! It leaves me with much more free time on my hands to do other interesting stuff as I won't repetitively need to water the plants everyday! But then, the story line keeps changing and is active based on how you play it. That's what kept me going after all :) In back to nature, everything is done after a year or two when you become super rich and feel like you don't have anything else left to do but Leaf Valley will keep you busy with plenty of choices :) Being able to ride the horse since from the start is very time-saving in the leaf Valley because it takes a lot of time to walk around the town whenever you have to go shopping or feel like wandering around town. Having to walk in Back to Nature until my horse was big enough did really take up so much of time which was annoying. Oh, and it's very easy to feed the cows and horses too. All you need to do is ring the bell to bring them out and then ring again to drive them back in on rainy days. Less of a burden really.
Anyway, I'm even buying a second copy :D in case my first copy no longer works due to playing it so hard.