The whole lore section is HUGE! The introduction of the Imperium is much larger! I don't have the 5th edition any more but from what I remember they didn't talk about different Imperial Planet types or go into great details when talking about the Imperial Hierarchy! In the 6th edition, accompanied by amazing artwork they go into much more detail! They have a large section talking about the different parts of the Imperium, planet types, the tithes required, the Eclesiarchy, Psykers and Mutants and the Time Line part seems much larger (and/or much more fancier) then 5th edition.
The Races part is not like the 5th edition - you don't get the 1 or 2 pages with the map of where they come from and some information and stats. In 6th ed the Forces of the Imperium gets a large section with a lot of information about the different organisations filled with more artwork too! Also, there is many sections describing wars like the Armageddon wars or the 13th Black Crusade where they show maps and tell you have many forces are involves (down to the number of Departmento Munitorum Engineer Corps involved).
As for the different races, they each get 1 or 2 pages of fluff about them (I think it's pretty much the same as 5th edition) but some, like the Orks, Tyranids and Imperium each get a page or two dedicated to their weapons where you see the artwork of many of their weapons like a Force Sword, Power Claw or Rending Claw etc.. Other Races get some artwork or some different colour schemes of their 'Kabals', 'Chapters' etc.
One thing that I liked is some random tidbits of personal stories that appear through the rulebook - like one is a story of a Navigator going into the warp where you basically read his inner thoughts as he goes into it. Another one is the story of a Chaos Space Marine Battle-brother who just finished a fight against Blood Angles. Others are very small and few sentences long info 'squares' about, for example, the First Contact with the Tau (there is a part about the Imperium wanting to know how the Tau are so organized and how the Ethereals manage to keep everything in tact where they say that Psychic abilities have been ruled out because the Tau are warp-immune - no protection under the Imperium though I'm afraid).
Chaos also gains a fairly large part and just get more information.
Later in the 'Battles' section of the book they go over many new missions you can decide to play and each one is linked to some more or less historical battle - for example a battle that occured at 993.M41 where Chaos Daemons infest some Starfort, eradicating its defenders but then the Grey Knights launch a counter-attack. They then go to explain about what has happened and they talk about different parts of the battle and then tell you how that sort of mission will occur (you can obviously use any races you want but that was just one example).
Another example is an absolutely massive fluffy battle report campaign (written in the form of a story) where a massive Chaos SM and Daemons force invade Eydolim (where, apparently, human life has first originated) and they go into explaining the different battles that occurred in the sector but focus on a counter attack the Imperial Guard and the Ultramarines have launched to defend the main city in Eydolim and what happened on that massive battle (accompanied by a huge quadruple page showing the whole battlefield!).
Finally you have a large and improved appendices section where you have a whole appendix with small articles explaining about things in the 40k universe like The Cult Mechanicus, or the High Lords of Terra, what I thought to be quite the hilarious article about field dressing a las-wound and more! Once again each article is rather short (from around 3 to 9 medium sized paragraphs) but it gives you a lot of insight!
Hopefully I've answered your question and hopefully I haven't gone passed the 'illegal to publish' line :P If I have gone passed that line tell me and I'll fix it :)
I loved the 6th edition mainly because of the amazing lore section and how they added some small tid-bits of some information about a battle, or a gun [like the Baal-pattern Boltgun with Blood Angels Markings painting that followed with a prayer], or some beautiful artwork (e.g. Terra's Sanctum Imperialis, or a squad of Blood Angels having a last stand against incoming daemons on a hill) that gives you some insight into the universe and the lives of the organisms living there!