This is a series of lecture's, not a one paragraph reponse...but I'll try
*Is there prospect pedigree? McDonald has it, Norris doesn't.
*Did something meaningful change with the player from previous years to this year? McDonald finally honed in on his control and got away from LA where he was horribly mismanaged. Nothing changed with Norris.
*Sample size - if there is a large enough sample (500 IP is a good baseline for a starter) saying a pitcher doesn't have what it takes to be a horse and their early season numbers suggest otherwise, trust the large sample
*What do the behind the stats periphs say about the pitcher? Fangraphs is your friend - LD%, GB%, FB%, K%, K:BB, HR/FB%, LOB%, BABIP, SwgStrk%, pitch value, pitch use, velocity. These can't be analyzed in of themselves and this is the part that takes longest to get used to, i.e. some pitchers just have stronger BABIP's than others - notably ground ball pitchers - so you can't use a baseline BABIP to analyze a pitcher you must determine if their specific BABIP makes sense given the type of pitcher he is. However, by knowing the type of pitcher that he is, viewing his past data, and accounting for any changes in the last year or 2 you can get a good idea if a correction is on the way or not. Gavin Floyd is a good example - stat heads love him but his numbers never match the behind the scenes data because he can't pitch with runners on base and he gives up too many homers. Put the two together and you have too many 4 IP 8 ER performances to offset the dominant ones. To own him you have to have the ability to guess when he's on and when he's off. As a Floyd owner, it's tough, but I have missed 3 of his blow ups and only 1 of his gems.
I don't have an explanation for Dickey. He doesn't know where his knuckleball is going and neither do the batters, but this year it's going over the plate a lot more than the past. I'm less confident he maintains this for the long term than McDonald, but he's in a groove now and won't be derailed until he hits some speed bumps - no clue when that will happen but it wouldn't surprise me if it isn't until next year.
Follow me @mchamberlin32