Disclaimer: I’m by no means a math expert and my statements below might not be 100% accurate, but I try my best. Also, be careful when tuning the rank profile as changing numbers can have a big effect on your ranking.
With the new FAST search core, ranking has changed quite a lot from 2010. Newly published content on
MSDN explains a bit more how rank is calculated and how you can change it.
As the O14 rank model is available in SharePoint 2013 (O15), I will try to outline some of the major differences you can expect to see regarding how results are ranked/sorted by default.
You can pull out the rank model xml yourself from both models using PowerShell.
$ssa = Get-SPEnterpriseSearchServiceApplication
$owner = Get-SPenterpriseSearchOwner -Level ssa
$o15 = Get-SPEnterpriseSearchRankingModel -SearchApplication $ssa -Owner $owner -Identity 8f6fd0bc-06f9-43cf-bbab-08c377e083f4
$o15.RankingModelXML > o15.xml
$o14 = Get-SPEnterpriseSearchRankingModel -SearchApplication $ssa -Owner $owner -Identity 9399df62-f089-4033-bdc5-a7ea22936e8e
$o14.RankingModelXML > o14.xml
Then it’s all a matter of comparing the models.