market potential

High entry barrier Corporation require large amount of funds most of the times and that high entry is because the technology or the costs. Even for acquiring the technology you may have to shell out huge amount of money. Most of the times in these kinds of business the companies in the whole industry may be only two or three and that will mean you will have very well entrenched competitors.

The upside that the market is huge and even if you make a small dent in the market you will be able to survive.
Update Mar 2010 - I read a few very good articles on business and economy here. A must read for getting good information on any topic.

Each of this of course has its won set of challenges and the main one is to acquire the funding. Low entry barrier business most of the times do not require much funds and hence you can easily get financing from people around you or banks. But for large barrier businesses you will have to have a sound business plan and a business team in place to approach big banks who may then be willing to lend to you.