Very simple: Older sets are more collectible, newer ones are more playworthy. Simple as that.
Now, for those of you who enjoy reading, let me elaborate.
I feel that the older sets are more collectible and hold much more value to many fans. The whole "vintage" feel of the old ones, some nostalgia blinders on max, and the actual collectibles included make up for some serious sentimental value. For people (like me) that like to have stuff to collect and display, this carries some serious value which I (and probably most of you) enjoy.
However, I feel the new sets are currently more poseable and playworthy. I mean, a kid can pick up one of the new 2015 sets and have a lot more fun with it (objectively speaking) than they can with a set from, say, 2001. While this is good, they don't hold that value described for the older sets that many appreciate, and to those of us used to the value those sets carry, feel slightly... well... cheaper.
Now, don't get me wrong. I love the new sets, and they are right up there on display next to my 01-03 collection, on the top shelf. I own every one so far, and plan to get the summer wave too. They really are well made, and while some are better than others, they are all exceptional sets. But, I still value the older sets more than the newer ones, and that is just me. Anyone that happens to value play value and stuff over collectibility, will value the newer sets more. But for many, they will find mroe value in the collectibility offered by older lines.
Personally, I'm slightly more neutral. Sure, I prefer the older sets, but that doesn't mean I don't love the new ones.
What do you guys think? I'm curious.