...Hearthstone is so popular, with so many people playing, that when a new expansion is released, the meta is "solved" too quickly. This leads to long periods of the "stale" meta.
I'm not sure what the solution would be. Do you have to perfectly balance each expansion before its released? or do you make nerf's much more quickly to tweak it back to stability? or do you just dump new content into the game twice as fast to keep things fresh?
I agree--this is a real problem with the game for me, as it discourages people from deck innovation and trying new things. My favorite aspect of HS is making new, unusual decks and running into them. My least favorite aspect of the game is running into the same decks over and over again.
I think one really key solution would be to stop releasing cards in "expansions", which is a holdover from the way physical CCGs release cards. There's no reason to do this with a digital ccg. Why not release a half-dozen (or even ten) new cards every week or two? Put them in packs of The Latest Cards, with cards cycling out after a couple months, where they could then go into more thematic "expansion" packs. In this way, there are always new cards coming into the game, and the meta won't get stale.
Another way to tweak the meta would be to limit the number of times a card could be used in a given time period. Each Legendary, for example, could only get 5 plays per day. This alone would have effectively dealt with the Reno nonsense.
And yes, as you suggest, nerf things more quickly.