Choosing a Washington outsider will give the Republican Convention a chance to mount a new line of attack and to blunt one of the main lines of attacks that Obama/Biden has settled upon.  McCain/Palin will attack Washington itself -- the whole corrupt establishment of both parties.   Not only will they  attack the democratic ticket, they will also attack the democratic house and senate.  They will  take some audacious jabs at both his own party and his own president -- though he will do this more in sorrow than in anger.   They might even allow that the Republicans deserved to lose the 2006 Congressional elections.   Or better  they will say something to the effect that  "we lost that election because we lost our way. "
This and other recurrent memes will be meant to decisively answer the charge that  John McCain is running for the third term of George Bush.  
In other words,  McCain/Palin  will both call out and call to their  party.    They  will call out the party for its past failures -- again more in sorrow than in anger.  And they will call to the party to be better ---  to joint them in a great crusade to remake the party, to remake the government, and to remake America.  At the same time,  they willboth  remind the audience in and out of the hall of the highlights of Republican ascendency.  The Republicans  will be represented as the party that stood steadfast throughout the cold war and presided over the ultimate victory.  They will talk about Repugs as the party that will win the  war on terror, etc, that tame the tax code and on an on.  They will say that they are the party that will reshape the American economy for the 21st century or some such thing and lead us to energy independence. (Drill! Drill! Drill!)
They will or course  make an explicit and strong appeal to democrats of good will.  McCain will highlight  the times when he has stood up to his own party and reached across the aisle.  Palin will echo this message.  We will learn a helluva lot more about how she took on the Republican establishment in Alaska -- a lot more.  We will hear about how she too reaches across the aisle.
The social issues will be handle lightly -- mostly through the personal narrative of Palin.  Somewhere we will hear about her decision not to abort her down syndrome child.
We will still  hear the relentless  attacks on both the readiness and the emptiness of Obama. Both Obama and Biden will be present as mean of many words and few actions.    Obama will be savaged as a man that has led no great fights, won no great battles.  We will learn a lot more about his habit of voting present in the Illinois legislature.  He will be called an absentee senator, who after 18 months in office decided to spend  his time in office on the campaign trail.  
This will be audaciously contrasted with Sarah Palin.  She will be presented as a reformer, a woman of bold, decisive, courageous action.  
McCain may even do something completely out of the box.   He may name his national security team.  And at least one democrat -- probably Joe Leiberman -- will be a member of that team.   
They  will paint a scary vision of what a unified democratic government will be like.  It would leave the democrats unchecked with untrammeled power.    A McCain administration would be dedicated to working with the democratic congress when possible, but it would also have the backbone to put a stop to it when necessary.  But this will be more subtext than text. 
A big play will be made for women voters, obviously.  But this will again be done through the personal narrative and character of Sarah Palin.  They won't hit hard on divisive social issues.  
If they pull this all off,  it will be game on.   And we will be in a tough fight.   The one thing that's to their advantage is that they will dominate the spotlight and can roll out Palin and their new narrative of the campaign in a relatively uncontested manner.  
The thing they have going against them, is possibly Palin.  She has to perform extremely well.  She has never ever been on a stage like this.  It's like bringing up an untested AA pitcher to be our starter for Game Seven of the World Series.   High chance of failure.   Plus we all know that McCain is no orator.  So I don't look for his acceptance speech to be anything to write home about.  
Who knows if I'm right about all this.   But time will soon tell, won't it?