Discouraged, encouraged... it all depends on the DM. If the DM is fond of using undead and other monsters immune to critical hits, rogues and keen improved critical scimitar/rapier wielders are not as effective as the greatsword wielding barbarian, effectively getting discouraged.
If the DM is fond of using high-mobile hit and run enemies, the full-attacking melee tank in heavy armor will not be too effective compared to the monk.
If the DM is fond of just using random monsters out of the book without regards to party capabilities, then the new DR may discourage certain characters. I, for one, don't play like that. I check what my players want to play, and then model the enemies after their characters - I don't take the "baseline" and force them to adapt or die. There may be the occasional creature or enemy that is very hard to hurt, but all in all my players' PCs will be effective in their chosen roles - no matter what it is.
If the DM is fond of using high-mobile hit and run enemies, the full-attacking melee tank in heavy armor will not be too effective compared to the monk.
If the DM is fond of just using random monsters out of the book without regards to party capabilities, then the new DR may discourage certain characters. I, for one, don't play like that. I check what my players want to play, and then model the enemies after their characters - I don't take the "baseline" and force them to adapt or die. There may be the occasional creature or enemy that is very hard to hurt, but all in all my players' PCs will be effective in their chosen roles - no matter what it is.