A great debate amongst SEOs is the question of links - should you target high quality, high authority links in small numbers, or get hundreds of paid links elsewhere on low page ranked sites that might hold less weight individually? Well, it's a tough question, but a very valid one.
Unique domains are undoubtedly a great source for links, and important for your site to gain page rank - with the right targets you can boost your ranking for certain keywords and niches qite quickly. But you may still find that however many dozens of links you get, whether they be manual or automated, you're still not gaining that positive ranking you want - and it may be costing you money.
Sites can beat you with less links, but higher quality ones. This is because links from high authority domains, from blogs or news sites for example, can be substantially more effective for your ranking. High PR sites can give you great link equity - sites with lots of juice and not too many links can pass on more authority to your site. In the same way a link not too far from the site's homepage in terms of navigation - one or two clicks beyond the home - are more likely to be beneficial.
This is why unique content and engaging articles are essential for your site if you want to really push up the rankings - you can't buy links off the BBC website or Yahoo News, but make a unique discovery or post an interesting article or tool and you may well find you the link building can run itself!