Nero Changed everything!!
1, Kirk will be a different person all together!, Imagine Growing up all your life with a Great Friend, a Mentor, a massive influence on every second of your life! - Your Dad! Kirk had a fantastic relationship with his Dad in the original time line, he was the reason he entered Star Fleet, Okay Kirk was still a slightly egotistical control freak but he knew how to take command perfectly, never second guessing himself etc.. Now take that Influence away and have you life controlled by an Emotionally crippled Mother who just wants to protect you, and smother you, and you'll probly turn out a little different!
2, Spok Explained why he's different!, Imagine watching your Entire planet being destroyed and every person (bar 10,000ish) wiped out in a split second, and Your Mum die in front of you! I'm not sure you'd be the same person you were before that event!, You'd question yourself at every move!! and Controlling you Temper (especially as you'r only half Vulcan) well I'm not sure I could!!.
3, Onto Earths Defences - I reckon Admiral Marcus told Star Fleet to 'Stand Down' or summut! he was the Bossman!!,
But Earths Defences have always been a joke!, In Best of Both Worlds, Apart from the Battle at Wolf359, Once the Borg Cube broke through to Earth there was only 3 Defensive ships attempting to hold them back as they passed Mars! Ve'ger in The Motion Picture, The Probe in Voyage Home all got past to earth without any interference!!