Having done this before, one of the obvious big challenges is establishing credibility. Getting a well known industry name on board can make all the difference - perhaps there is one firm that you have a relationship with at some level that you can persuade to be an initial customer? - perhaps by giving them a really good deal or doing some specific customisation work just for them? - if you can solve a real business problem of theirs at fairly low risk to them, then you may be able to get them signed up and agree to do a written case study. Assuming you can afford it, getting this client live and happy is worth doing at zero profit (although don't give it away as nobody values free stuff).
Once you have a "household name" in place things generally become a lot easier - objection handling during the sales process around your size or immaturity is much less work. One suggestion: read Crossing the Chasm - it describes how to get early adopters for your product, and then make the move towards mainstream adoption, suggesting strategies for focusing your early marketing efforts most effectively.
One final point. Again, if you can afford it, go for a subscription pricing rather than up-front licence sales model. I have found enterprises to be quite receptive to this - it shows you are in the relationship for the long-haul, and it allows them to predict their long-term costs (rather than have to worry about potential fat upgrade costs every couple of years). Although this means less money through the door in the early stages, it allows you to build a much more predictable revenue stream year-on-year, and it forces you to maintain dialogue with all your customers on an ongoing basis, reducing the chances of them switching to a competitor.
Selling enterprise software is not easy as you directly face big giants of the industry. The basic challenge is building the trust. Why should an enterprise trust you when there are big players in the market? Please be careful in this area as it can become extremely difficult
You will want to have sales folks on board who have sold to the target audience before. You will also want to leverage VARs who have a trusted relationship with your target audience in place and you will need to motivate the VARs in such a way that there is (1) differentiation in your offering vs status quo today and (2) more money for the VARs to make for themselves through your discount structure or consulting that they can sell around your solution, in one way or another. There's a lot to this, and you'll need help from people who have done it before to pull it off if you have not done it yourself, generally speaking.
You may want to start with smaller companies, as they may be too small for the larger competitors to worry about, but it will give you some clients you can point to as a success, and you can work your way up to larger companies.
As you develop some case studies for your product then you can use that to help show what you can do.
If you feel you are ready to make the jump to larger companies, you may want to sell to smaller companies for some income, and target some well-attended conferences to showcase your product, but you should have a really polished approach at that point.
Lots of valid points, but you better be ready to put together a support staff. Installation, configuration, setup, training, customization are going to be a large burden. Having the necessary infrastructure is going to be expected (See Siddharth's comment on Trust). You may work with VAR's who are tech savvy, but they will expect all of your updates and documentation sooner.
Have you worked with other companies in this area? Maybe you created a custom app for one client and are considering selling a version to others.