Without proper planning, there are two main problems that are likely to be encountered.

Luckily, both of these issues can be avoided by use of a simple solution called balance. I've found my own little sweet spot that I feel avoids these pitfalls. I find that a width of around 950px tends to work well with the current crop of common resolutions (It is common practice these days to use the 1024 x 768 resolution as the lowest common denominator). Using creative backgrounds and having the headers and footers span 100% of the browser window help eliminate any excessive emptiness issues caused by the ultra-wide resolutions. Another tactic is using flexible "stretching" layouts that shift according to the browser window size, but I find this often leads to results that remind of the table based layouts of the 90s.