You can see for yourself the changes to the "Hub and Spokes" rules, as they are the first 90 pages, and are included in the preview at http://www.burningwheel.org/?p=276. Under the word "Here" in the fourth paragraph on that page is the download link for that preview.
Luke has mentioned that there are numerous small changes to Fight, specifically the processes for range setting, Duel of Wits, and Range and Cover.
Sorcery no longer adds will dice to rolls to cast, reducing the difficulty of advancing Sorcery as one no longer needs insane Obs to get Difficult and Challenging expenses.
Perception is no longer open ended.
Let it Ride drops the bit about referees cheating.
A significant change in terminology: "Graduated Test" replaces the older "Open Test", avoiding the frequent confusion between open ended rolls and open tests.
Some minor changes to the difficulty/test type table at the low end.
Many lifepaths have been revised.
I've not yet gotten BWG yet, only the preview, so I can't be much more detailed, but I'll note that the hub and spokes text is not readily different at first glance, but some small changes are present.