Challenge system
The challenge system is everything but fair. First you learn one or teo languages and raise your exp. Then you do challenges. You don't do challenges before you learn, right... and beginning with an new lang on a higher level always lets you loose XP because you need to start vs. people that get the easier challenges at the beginning. It should be PER language and depending on your experience that you already have made. Example: you begin with 2 o. 3 courses for languages like Ruby, JavaScript and SQL. Then you make many challenges till Level 14 or so. If you now begin with a new language you get nearly no XP from challenges in that new language against lower people an loose massively XP (gathered with you other skills) by getting challenged by lower people although they are based on the new language on your level or even higher.