I have to disagree.
After reading all the answers and thinking about it i can say that concentration in itself may not be so bad an idea. But it should not stay the way it is. What a system like this needs is consistency. If you compare spells and wonder why one needs concentration and the other does not, something is not right. That maybe put a bit too simplistic
but i hope you get what i want to say.
If you have spells that are very similar in what they do and how they work but one needs concentration and one does not then that will surely lead to questioning.
As of now the amount of concentration checks seems way too high. I really would rename it to "maintain". In a way that it needs magical energy constantly poured in to keep it active. So that falling prone and other little diversions can be ignored and conc. checks for this can be removed and you just need to check when someting really impacts the caster.
