Getting to level 10 is no major achievement, but it does come with some benefits, most notably your first talent point. Being a warlock for the last 9 months or so, and having respecced several times, I can tell you where my first talent points go...Improved Corruption 5/5. For non-warlocks, that means I can instant cast my corruption spell, meaning I have 2 instant DOTS ready to go at any time. I find it crucial, and even more so if I happen to be on a PvP server. I have tried speccing without it, but when I see the casting bar start counting down when I click the corruption button, I feel as if I have gimped myself.
So, what is absolutely the one talent you can't live without in your class of choice? Is it an easy one, like the Improved Corruption, where you can spend your first five points on it? Or is it deeper in a tree and requires some fancy planning?