Let's see how decks will be compared to pre-nerf:
Hunter: Obviously pre-nerf it was pretty much the strongest deck. Probably will still be viable, some people that actually really like Hunter will probably work with it a lot to make new decks, I expect Deathrattle Trap decks to be good. Maybe people will try control Hunter, that'll be fun. But it will definitely be one of the weaker decks overall, other than its niche in countering Handlock and stuff.
Rogue: Miracle was also one of the strongest decks pre-nerf. Also probably won't be that strong afterwards. Backspace Rogue might get play for doing well against Handlock, and Malygod decks or Forsen's Southsea Deckhand ones will be used by people that just like Miracle, and the matchups will basically be the same, but it won't be a super popular ladder deck since it'll also be weak to Handlock and stuff.
Warrior: It was good pre-nerf because it had a playable matchup against everything, and it still does, but before Hunter was the meta and Warrior was one of the best choices against it, whereas now decks like Shaman and Paladin will be much more popular, and those can be difficult for Warrior.
Priest: Was pretty good before because it could handle Hunter and Druid and Warrior well, but Priest has a tough time against Handlock because it's just not fast enough to beat it and can't handle of the threats (Twilight Drake ;_;). Also weak to Paladin, but it's really good against Shaman, so will be decent.
Druid: Was average before, and still is decent against most of the field. The obvious weakness is Handlock, and that'll be rough, but just bust out your double BGH decks and you'll be good to go. Druid's also nice for being able to burst Handlock from out of Molten Giant range, so it's definitely possible to win by just playing around them. I think there'll be an even mix of midrange, token, and ramp builds.
Shaman: This was basically dead pre-nerf. It is one of the best decks against Handlock since you always have Earth Shock in your opening hand for the Twilight Drakes and Hex and Flametongue for Giants. Also people will just be happy to play it with Hunter and Priest less prominent. Good anti-control deck.
Mage: Was hard to play with Hunter, although a lot of people did try stuff, and it could be good. The aggro build is pretty good against Handlock since it can reach through big taunts, and control decks could be really annoying. Giants Mage is scary, and the degenerate midrange secret builds will be hell to play against. Seriously, Counterspell is stupidly good against Paladin because the only throwaway spells it has, which really aren't throwaway spells, are Humility and Holy Light. And of course they can run secrets without the fear of constant Flare.
Paladin: Pre-nerf, and for a while, has basically been the least popular class or the second least popular ahead of Priest pre-Naxx. It did actually have a lot of good matchups and was popular in tournaments, but people just didn't play it. It was tough to play against Hunter, but that matchup was decent. Now, it's really good anti-control. This is my main class, so I'm excited. It's a good matchup against Priest, Handlock, and Warrior, and its weak matchups, like Miracle and Hunter, are out.
Warlock: The big one. Zoo is and always will be a strong deck and necessary for the metagame to make sure that decks don't become too greedy, and it doesn't really care what the meta is, so it'll be fine. Handlock was basically dead on ladder with Hunter (although I found that it wasn't a terrible matchup) until a few days ago. Then it got really popular, mostly because people were playing more anti-Hunter decks than Hunter decks, and Handlock beats all of those. This is the new Hunter, the deck that the meta revolves around and tries to beat. Should be fun.
Also, if you guys are looking to replace Leeroy, there are many good options. In Miracle, you can use Malygos or Southsea Deckhand, or you could be really pro and build Murlocle Rogue and Shadowstep Old Murk-Eye for the hilariously effective wins. In Handlock, you can run Malygos and use double Soufire for 18 damage and then cry as Soulfire discards the other one every time.
TL;DR Handlock is king, Shaman and Paladin are good for beating it, fuck Mage, Druid, Priest, and Warrior are alright, and Hunter and Rogue are weak