hmmm... I like it as a class feature, but wasn't this was supposed to be the warlock's shtick? Actually, it's better than the current warlock, as he is forced to take Eldritch Blast as one of his two at-wills. I think I can almost smell an errata...
And Arc Lightning just robbed the greatest advantage of the pre-errata Magic Missile: Range 20. It's like it killed MM, took some of its stuff, and then MM reincarnated with automatic damage...
Either way, I approve this. In fact I'm hoping them to errata most Wizard at-wills to increase their range, so they can better control the battlefield. Ray of Frost should be a priority in this case.
As for the Essential Spellbook, it doesn't mention anything about adding new spells to it. I hope - dear god, I
hope - they are not limited to two spells per level, becoming much more like adding rituals. It's all magic, right? why should they be treated differently? (I know, I know... "balance" :... [rant] I say screw balance! I want to collect enemy spells! I want to have the right magic attack for the right time! [/rant])