Users are not only giving to a charity and choosing the portion of the price they pay to a charity but they can also select from a list of charities to donate to. Thus giving some control to the consumer as to what kind of organizations they support.
The way Humble Bundle works is that it is donation based in that you can pay what you want for the bundle, with the minimum being a dollar. There are separate packages in each bundle that also ask for a minimum price, which depends on the "beat the average" price which is what on average has been donated so far..
The bundles have the support of the devs to be published, which means there may be certain devs that do not want to have their games in a bundle, which limits how many games are available on their store and in their bundles.
Just as a user can choose how much to give to a charity out of the price given, they can also choose how much to give to the devs. So if one does not feel like supporting a charity for a certain bundle they can choose to give all of the funds to the developers.
Currently there is no option to filter reviews for a game as it would be easier to see poor reviews on their own over seeing the score mixed altogether.
GMG often offers discount codes on newly released games or pre-release games. For those looking to get a new release GMG is a good place to look as users can often find a discount of 15-20% off the game instead of purchasing at full price.
GMG does not offer refunds on Steam, UPlay, Origin or MMO titles. Basically if you purchase a code to a different store, GMG will not refund your money in case of issues, which means one will have to deal with the keys origin store for help.
Rather than trying sellers blind, you can see what the community has to say about both sellers and deals, as well as comments saying if you can get the same options cheaper elsewhere.
Due to the large amount of users (355k) this allows for a constant updated list of all the sales on games across the internet and box stores to be listed and updated very frequently.