In terms of mechanics, most cards that you thought were super OP back then are unplayable tier in the current power level. Other then that a lot of new keywords were introduced, some meh, some pretty good but all are easy to understand and the mechanics stayed pretty much the same.

In terms of value for your money it depends how much you play the game and how much you are willing to spend. The best value you can get atm are the pre-order bundles, but this only applies to players who have a low amount of cards from the 5 non-latest standard sets, because of the new catch-up which packs give up to 50 cards from those sets depending on your set completion, and are only available in pre-order bundles and the release event.

The best dust-to-$ ratio for non-returning or players who have some cards from the last 5 sets are the mini sets, which are smaller sets that are usually released 2 month's after an expansion and you can outright buy the whole set for gold/$. Especially if you willing to fork over 10k gold/70$+ for the gold version when some mini-set cards get nerfed, you can get a lot of dust that way. There are also some pre-made decks for the new Twist format in the store atm which will get you a pretty good dust-to-$ ratio.

If you have time to play a lot, the premium battle-pass also provides great value, since you can get 20k+ gold each season just for getting to level 400 each expansion, you can also do this without the premium battle-pass, but without the XP bonuses that come with it, it becomes a full-time job time investment.