This is one of those title says it all posts, but I'll elaborate: the fact that there is neither a new class nor a new race in Warlords of Draenor isn't a problem. First up, there's the obvious fact that we're getting redesigns for the eight original races plus draenei and blood elves. In terms of art design, that's an incredible amount of work, far more than designing one or even two new races. Racial abilities for each race are also being redesigned, meaning each will play differently. Moreover, by not introducing a new race or class, we don't need to have a starter zone designed for them, meaning that content design can focus on content for the 90 to 100 player, especially since thanks to the level 90 boost, it can be assumed that anyone who picks up Warlords and wants to play it can.
As has been said elsewhere, new races and classes are not content in and of themselves. They consume time and development resources to create them, and often they have content associated with them, and that content is usually only playable when you create one of them (although the monk did not actually get that treatment - save for one location in Pandaria that offered monk only quests, as a kind of home base, monks didn't see the death knight starter zone style experience) but by themselves a new race or class is just a different way to experience content. This is not to say they are not important. New classes offer new gameplay options, new abilities and spells, and sometimes new roles for players who did not enjoy, say, tanking or healing on previous classes.
But I think it's fair to say that World of Warcraft doesn't need the added complexity of three new specializations to balance right now. There's going to be a lot of work needed to balance out new spells and abilities, adjust item levels, change the way healing works while ensuring it does still work, implement entirely new gameplay like garrisons without also figuring out how to keep another class in the mix with the other 34 specializations we already have. Similarly, while I mourn for my alliance ogre paladin and horde arakkoa druid, do we need two more groups of racials to balance out?
If anything, I'm somewhat heartened by the lack of new races or classes. For one, it means draenei might actually get the story focus we've been promised since The Burning Crusade came out, which I'm ridiculously excited by, but moreover it means that almost all the content we get will be intended for the 90 to 100 player. You won't have to roll a level 1 and play through between fifteen and twenty levels of content to see it - your main, who you won't have to switch to a new class to check out the new hotness, will get to roll into Warlords and start playing and experience everything. No side tracks, no being told that I should now roll a low level undead to see the other half of the story my worgen just experienced, just content for everyone (since with level boost, everyone can be expected to be able to experience it when it drops).
One of the biggest problems I have with low level content that comes in with expansions for the new races and classes is how often it ends up forgotten, both by players and developers alike. Gilneas was an excellent zone experience, I freely admit that - and now it's gone, and as soon as you get out of it you're shunted off to Darkshore and the game just kind of forgets it happened minus a forsaken zone storyline in Silverpine that just makes it worse, really. We got a mention of it at the end of Mists of Pandaria's ultimate raid encounter, but lets be honest - that doesn't feel like anything.
Even when the quest content isn't sealed away behind a 'you must be this race to enter' wall it ends up mostly forgotten - the events of Azuremyst and Bloodmyst Isle are still pretty epic to experience, but it doesn't have any real effect on the game and it's seventy levels in the past (and getting more distant with each new expansion). It's content forever separated by gulfs of time from the other content in the expansion it's a part of. I think, even if you dislike blood elves, the blood elf starting zone will help you understand them and their experience - but it's forever divorced from the now of the game, and forever in need of updating. Will blood elves report to Vol'jin now? I assume so.
We have the opportunity in Warlords of Draenor to get an expansion that does what we haven't seen in an expansion yet - namely, we can have an expansion that entirely focuses on players at the max levels. All the content for Warlords can be designed from the point of the last expansion's level cap and move on from there. There's no need to redesign old Azeroth, no need to develop twenty levels of questing content for a new race or build a new zone with compressed questing experience for one class. Everything we get can be designed on the assumption that everyone will be this tall and can board the ride. The effort that's been expended designing excellent zones and stories that you only get to see while blasting through it wearing heirlooms can now be expended on quests we'll get to see on our mains, at the speed one levels at for the first time. I'm excited for it.