As a noob, I find that my biggest mistake while hand registering domains is my impulsive and compulsive behavior. I do my research first of course, but when Godaddy tells me "Hurry and get this domain before someone else does" ...
I'm really reading:
"Hurry and get this domain right now because there is someone else doing the same research as you and they are about to buy the domain too so you better buy it right now .. stop looking for Godaddy discount codes that work because there are none left, you already maxed them out, you're wasting time ... buy the domain now ... hurry the heck up!"
Then later, after I'm happy with my domain I bought, I realize I need to buy all the other variations of the domain name to keep (my perceived) value high.
I still feel positive about the domains I register, they are in technologies that have yet to make it big, but sometimes I look at my domains in horror at the future renewal costs and with doubts if I speculated correctly.