So I heat and whisk some cream with egg yolks, sugar, and lemon juice in a water bath until thick, whip the rest of the cream, mix the two together and put it into a freezer. After five or so hours I take it out of the freezer and it's done. It's the recipe from my ice cream book. Four yolks per 500 ml of heavy cream (33%). But it's not smooth, it has that "ice crunch". I can feel and hear those tiny ice crystals being crushed by my teeth (it's quite subtle, but it's there). How do you prevent ice crystals from developing in your ice cream?  Fat must be the key, but I had plenty of it. Besides, you get no ice crystals if you just freeze whipped egg whites with sugar even though it contains approximately zero fat 
The key points of avoiding the formation of ice crystals are primarily dependent to the speed of the freezing and the amount of agitation. Commercial batch freezers have more powerful compressors that only take 12-15 minutes to complete the process, while ice cream machines for home use tend to take 45-60 minutes. Also a higher dasher speed, with up to 200 rpm, can help to gain smaller ice crystals, but also results in a higher overrun, which might or might not be wanted. A detailed description of the freezing process can be found in the Ice Cream Technology e-Book by H. Douglas Goff:  Theoretical Aspects of the Freezing Process  Structure from the Ice Crystals   