rfd,
location, location location is the best starting point. The next thing is the specific amenities of the land (water,pasture,timber, etc). People talk about land as a generic but it really isn't.
If I had to make generalizations, I'd say that generally, agricultural land out west is likely to be worth less over the next 15 years if you don't have the water rights to go with it. Predictions are that the draught conditions will be around for a long time.
Agricultural land in the midwest is going up at a nice clip because of high corn prices. I won't predict over the next 15 years but certainly over the next 3-5 I see it going up in value. Cash rentals are finally starting to move up compared to previous increases in land prices. This could set the stage for further increases in land prices.
On another note, if you are looking at it strictly as an investment you might want to buy land in the (Eventual) path of development.
Mobing right along, interest rates always have a big impact on land values. It's all about the carrying cost. Kiplingers finance has an interesting article in the August issue about investing in lands (they are talking about tracts in the 500 acre or more range). I think they paint an overly rosy picture but that's me.
You also need to consider the tax impact. If I had to pay regular property taxes rather than CAUV (Current Agricultural Land Valuation) my property taxes on the farm would be a lot higher. Because we actively work the farm we have a different tax treatment than if we were passive investors in land. Do your homework or hire a tax accountant that understands real estate, particularly land.
You also need to consider the impact of doing a 1031 exchange. If you can put off paying taxes (from the sale) on a property, that can be a valuable thing if you are looking at the long term.
Just a few thoughts.
Mike