when i’m going to start a new site, i either check for content or sponsors. when some people start a new site, they buy some domains first and then get to designing, but i’ve seen threads where a site can go wrong if you can’t find enough content or there are not enough sponsors.
of course the above is different if you’re shooting your own content.
one variation on above for me is if the site is going to trade traffic. the first thing there is to make sure the niche i chose will have enough sites to trade with. i’ve seen too many guys who had great ideas forced to either give up or come up with an entire traffic network because they needed at least 10 - 15 trades and there were only 3 or 4 sites to trade with in the niche.