Why is the 6D considered as an "enthusiast" camera while the 5D as a "professional" camera? I thought its the opposite. I need a replacement for my 20D
Mainly build quality and a few "features". The 6D is much newer but appears to be built on the 60D type platform (although it still apparently has a mostly magnesium alloy body). It believe it has slightly fewer dedicated buttons than the 5D2 and also doesn't have the joystick control. Shutter speed and flash sync I believe are both slower. The 6D uses SD cards while the higher end cameras use compact flash which may not offer any real advantages to the latest SDs except being bigger and more expensive.
Many of these features were present on the 7D which was a very well built (although aps-c) camera.
In the end I believe they're fairly minor complaints. My personal biggest gripes with the 6D are the poor AF system (single cross sensor, really? Again?) as its arguably poorer than 7D, 60D and even t4i which all now use all cross-type sensors. Single card slot is another let down as is the lack of a 100% viewfinder. These may not be deal breakers for everyone though. Personally I would rather have had all of the above instead of wifi and GPS, but that's me, for others different features may be important.
In the end, it's a $2100 camera with a lot of shortcomings. At $1500 after you factor in the lenses it's a totally different story! Nikon's D600 isn't perfect either, as I suspect both nikon and canon made compromises so come in at a certain budget and to not cannibilize sales of the 5D3 and D800.
At release the 6D was going for full price while the 5D3 had dropped to a street price of 2800-2900. Which to me makes the 6D quite in attractive. At the sale prices we've been seeing its a very good deal!