Reason for optimism: Sending Clayton Kershaw to the mound every five days induces a perpetual state of optimism.
Reason for pessimism: Replacing the results of one-half of a historic duo is daunting.
Kershaw and Zack Greinke gave up 107 total runs last year. Ignoring the handicap that they did it in over 65 starts and 455 innings, you could take any two MLB starters who qualified for the ERA award last year (i.e. minimum of 162 innings pitched) and they will have given up more runs than Kershaw and Greinke. Think about how astounding that fact is.