Its probably fitting the heatsink for me, though this was the first computer I've built, so thats probably why. I was scared it'd crack the chip, because I found out after about 5 minutes of trying, I was trying to do it the wrong way....and the lever wasn't clicking into place like it was meant to. One final push though, and CLINK. It worked
The second most thrilling should I say, is turning it on and watching things spin up for the first time, especially if you've just fitted lights etc, and just watching how it copes. mine did alright after I fitted the 12v connector, because I wondered what I had done wrong when it wouldn't boot
I thought I had killed the processor...