Take a look at most blogging platforms for ideas of carrots or sticks. I.e., you get a free blog, but can get a lot more if you pay.

I think your question boils down to four things:

What is core to enjoying the site?
I haven't looked at your site, but someone mentioned games. If you want people to come back, it should be enjoyable. If they come back and something they expect isn't available to them and requires payment, they might begin to resent you. If you're going to embrace the freemium model, I think most successful people have made as much of the core stuff as free as they can. I'm not saying make as many things free as you can, I'm saying make as many important core elements as free as you can.

What are your competitors doing?
Two things you should look for. One, what are they charging for that you're ok with giving away for free? Its a marketing tactic. You can get their users if you provide something better. There's nothing worse than charging for something just because you needed to pick something to charge for. People will look at it and say "Thats so simple, why isn't that free?"
Two, what do their users think about the things they charge for/give away. This is a competitive intelligence tactic. Go read their forums, and look for clues. Did they try and charge for something and have a huge uproar? Or did people just say, "sure, ok". What is the most requested feature they have? Its pointless to make your competitor's mistakes when you can see what the results were. Don't copy their bad choices. But to know what was bad you need to do some research.

How much does it cost:
I think most people are ok with paying for stuff they know costs you money, IF you clearly make that the case from the start. If you give away almost everything for free and then try to introduce cost into the mix, it doesn't go down as well as it would than if you had said upfront "Hey, this costs me a lot to provide, so its going to have to go into the premium side." You also have to think somewhat long term. Advertising won't pay for everything, and its likely that you can't keep covering all the costs. You won't be around long if everything is free. If something is very expensive to provide, ask people to pay for it.

Don't be afraid to change:
As your site grows and gets older, get feedback on what parts of your site people really use. You can switch stuff around from the pay or free columns and experiment a little. If there is one feature people consistently ask for to be included on the free side, and it hardly costs you anything to provide it, why not give it away?

A word about advertising. I may be in the minority here, but I don't care if advertising is on the site or not as long as its not annoying. I wouldn't pay to have no ads, because I'm not bothered by them. If I was bothered by them, I probably wouldn't use your site. Why don't you research how well that has worked for others. Do people actually pay to remove ads, or is it just something in a list of things to make money that nobody has actually bothered to research lately to see if it works? Almost every site we use today has ads, its just a fact. I say keep them across the site, make them somewhat inconspicuous, and use the revenue to offset the things you give away for free.