I'm not entirely sold on it (assuming I'm right! I could be wrong!), as it provides very little granularity. You're either skilled or you're not, with no in-between. The only difference between two characters with training is in their ability modifiers.
Originally Posted by JackMann
Their design philosophy leads to the opposite conclusion. The current (3.0/4.0) system leaves you either skilled or not (i.e. trained in a skill or not). That system encourages players to use skills they are trained in and ignore the other skills on the list--particularly during skill challenges. The new philosophy is that ability scores are not just raw talent, but a combination of raw talent and training. So a person with a 17 intelligence is not just a person who was born smart, but a person who knows a lot of stuff through study (or just a good memory). So the granularity occurs at the level of ability scores and is further emphasized with more specific bonuses and perks that come from backgrounds (i.e. the delivery system for skill bonuses) and themes (i.e. the delivery system for feats). Class is now divorced from skill training (i.e. skill bonuses), meaning that it is easier to mix and match class and skill choices (i.e. play a wizard with rogue-like skill bonuses) without the pressure to multiclass.
Ultimately, the goal is to use ability scores, rather than skills, for task resolution in the game. You want to climb a wall, you roll an ability check, not a skill check. But if you're background gives you a bonus to climb, you get to add that to your ability check. So, rather than a finite set of skills for which you are either trained or not, skills in 5e become a broader range of situational bonuses that modify ability checks (i.e. when you make a skill check to climb, you add a +3 due to your Background as a Cat Burglar).
To do so, they need to limit the range to which bonuses and DCs scale, and the degree to which bonuses can be modified by training, feats, and other sources. They don't want 1st level characters with a +14 bonus in the new system. Those kinds of bonuses lead DMs to create challenges (i.e. DCs) capable of challenging the guy with the +14, inadvertently placing the challenge (i.e. the DC) well beyond the reach of the other PCs in the group. By scaling down the possible bonuses and DCs, anyone can attempt to fast talk his way past a suspicious guard.
Here's a longer blog post
explaining the new conceptualization of ability scores vs skills with regard to task resolution.
Here's a longer blog post
about Backgrounds and Themes and how they lead to greater flexibility and more options in skill selection and character creation.