For me, it would have to be how relevant the domain is to its niche. Again, it may look trivial but you may also want to find out why the owner is selling the domain in the first place. I guess you have to greatly rely on forecasting.
The major reason we are in this business is to make money so I think your main concern should be about how much you can make from the resale. Is the niche ever green? Is the topic highly searched and so on.
In addition to the points mentioned above, I think it will be beneficial to know the prediction of upcoming trends. I agree when people say that trend is created; that includes anything on the internet. If I think the topic of the domain may get more buzz in the future, I would definitely consider seizing it now.