themes, the entire rf network setup is done on the remote, choice of favorite channel icons or commands showing first for activities that support them...those are some examples...I'm sure there are more I'm forgetting at the moment
keep in mind that the 890 lacks colored buttons that are common for BR players and sat boxes, which means assigning them as labels on the display
it also lacks LCD screen images for anything other than favorite channels
the 900 has a pretty good assortment of images that you can assign for commands on the LCD (note activity images are not customizable on any harmony)
having had the 880, 890, one and 900, I can say that the screen quality is the best on the 900...it really makes the 800 series screens look terrible (and I thought they looked great back in the day)
as far as the sticky ir blaster tip cables from the old extender, those can be plugged into the back of the new main blaster with the correct adapter (the old blaster cables are 3.5mm, the ports on the blaster are 2.5mm mono)
I do highly recommend the 900 with two caveats
1. the 890 supports sequences and the 900 does not
2. the 890 supports the uploading of a csv file for display of a media list for things like a DVD jukebox....the 900 (and any harmony lacking a physical "media" button) lack this feature....there is a nice harmony model comparison here that has images of all the remotes so you can see which ones have the media button and which ones don't
http://bevhoward.com/Misc/IR/HarmCompare.html
if those two features are needed by you, go for the 890...otherwise I would suggest the 900