From a technology perspective i would say the following would make a dream application.
first and foremost "ease of use". What ever gadgets or bells and whistles you throw at this thing, it needs to balance that functionality with ease of understanding and a logical and speedy interface. Not as easy at it sounds, especially for techies who do not usually understand who the end user of their application is and how their brain does or more importantly does not work.
Next is flexibility, i like the ability to search from any perspective, be it the candidate/applicant, the job or the client and in a variety of ways. Including skills based, cv text based etc.
Next would be solid website integration, allowing easy post of jobs to your own website and popular job boards and also allowing applicants to apply for those jobs and have that data back in your system
Also scalable costs that increase in proportion with your business growth rather than startup costs that cripple you from the outset. And watch out for long contracts, companies with good software should be confident that their product will compell you to stay not legal handcuffs.
There are loads more but these would be my key features.
I use http://www.applicantextra.com
but others are available.