How to determine what's turning people off about your service?
neovintage: I think you're looking in the wrong place. Who cares about what the consumers think who've decided not to sign up?  Personally, I don't think users know exactly what they want or how to convey how they feel about something. Dont get me wrong you need their input, but take it with a grain of salt.Before thinking about why people aren't signing up for your service, think about who SHOULD be signing up for your service.  What I mean here is, who exactly is your target customer for the service?  Does this person use particular websites, do they only spend a couple of hours on the web every day, do they like to use twitter?  (Since I really don't know what your service is I can't make better suggestions)  Basically, what are the characteristics of your target customer?  If your market is defined too broadly then your service doesn't really stand for anything.  If your market is defined too narrowly then you'll be running into the problem where you don't have enough people signing up.In terms of actionable steps, this is what I've done in the past for a business that's already running:
1.  Do some research on your current customer base, mine your current database.  Some questions to ask include:
- What websites do they visit before hitting your site?
- What websites do they go to once they've finished using your service?
- How long do they spend on your site?
- In a survey, how many people work at their company or how much revenue their company makes?  
- In a survey, ask how people heard of your site (subsequently visit those sites to see what context your site was mentioned)2. What patterns are emerging from the research you've been doing?
- The goal here is to find a type of user that have grown a strong affinity to your service.  Once you've found that strong user, it's on to the next step.3. What needs to change based on the patterns you've identified to attract more of those "strong users"?
- This is easier said than done, but you need to think about pricing, value proposition for user (what is the user going to get out of using your site), or are advertising in the right places.  
- Much like the others have said in this thread, A/B test is a good idea if the value proposition is off.
- You may want to consider advertising, if it resonates with the user.  This includes all the typical social media outlets.I hope this helps.  Good Luck!