I've been playing this game for awhile. Opened a bunch of packs on launch. Standard format and the expansion has done a lot to freshen up the game. I usually play competitively until I get to rank 5 (the rank in which you get a golden epic card for the end of season ranked rewards). Then I just play around. I've been fooling around with midrange shaman, miracle and deathrattle rogue, beast druid, control paladin.
In regards to dusting naxx and gvg cards, it depends if you have any interest in wild and/or tavern brawls. Some tavern brawls require you to build a deck from your collection and I believe you can usually use cards from all sets. Also, I'm pretty sure you will be able to use any cards when building decks for the new adventure sets.
For me personally, I haven't touched wild since standard came out. However, I don't have a huge need for dust. I was able to craft most of what I wanted without the need to disenchant any gvg or naxx cards. So I haven't dusted any cards. However, if I were to disenchant them, I would start by getting rid of some of the weaker legendary and epic cards from those sets and see how far that got me.