I would first start off by saying that i absolutely love my 3808ci and have owned Yamaha, HK, etc. and really have enjoyed my Denon the most. It sounds fantastic and the flexibility and user friendliness is probably up there with the best.
My reasons for upgrading are pretty minor actually. I am very interested in Pro Logic IIz, especially after I read a reviewers impressions in Sound and Vision last month who claimed "I own the gear for 7.1, but never really got excited about it but I am very intrigued to get Pro Logic IIz set up in my own home." Secondly, I own the 2500 transport and still watch a decent amount of SD DVD's. The FL 2310 scaling solution in the 3808ci isn't bad by any means but the anchor bay 2010 chipset is supposed to be superb. I may need to be corrected here but I believe it is the chipset used in Yamaha's Z7 and you can read the review from audioholics on the scaling as well as impressions of the new OPPO BD player which uses the same chip.
Overall I am that guy who has to have the latest and greatest but I won't discount the fact that I work in the industry so i get the products considerably cheaper than retail