Shimano 105 10 and 11 speed compatibility
If you want to go from Shimano 10 speed road to 11 speed road, you'll need a new cassette, new shifters, a new chain and a new rear derailleur, since the cable pull ratios have changed (again). Zinn also notes that 10 speed FD might not shift as nicely as 10 speed FD with the 11 speed shifters, so may as well throw one of those in. 

So, in the drive train, essentially all you'd be wanting to keep is the crank if you want to go from Shimano 10 speed road to Shimano 11 speed road.