Well, it’s cheaper, better stated, and affects all of your other minions.
It is important to note, you can throw this out on curve unlike Rhino and it will probably survive. That same curvestone mentality also reveals this on turn 4 into any level of Emerald Spellstone is extremely good at establishing board control.
And other than Houndmaster, what other 4 drop would you be playing in a minion based hunter anyway?

First off, there will be another Hunter legendary minion.
And also, this has better stats than Tundra Rhino, is cheaper than Rhino, and applies its effect to all minions not just beasts.
While you can’t throw minions at your opponent’s face instantly like you can with Rhino, Rushing your opponent’s minions for control of the board will make the Wolves summoned off of Emerald Spellstone at lot better than they already are, and unlike Professor Putricide, the statline of this card will make it far more likely to survive being played on curve.
Rush is not as good as Charge, but it will be good enough to get this card inserted into minion based hunter decks since other than houndmaster, there’s not a whole lot else to play on turn 4.

Wasn’t the same true about Priest’s spell line-up when Lyra the Sun Shard was revealed last year?

Wasn’t that the exact same reaction to Lyra the Sun Shard?

If Priest’s generally lackluster spell line-up wasn’t enough to stop Lyra the Sun Shard from being good, I doubt it’ll stop Hagatha from being good. I agree with you on the deck idea though.

While many of Shaman’s spells are a bit lackluster (Farsight, Frost Shock, Earth Shock, ect.) giving every minion the battle cry of “Add a random Shaman spell to your hand” is extremely powerful, not as powerful as draw a card though. If the shaman spells in this set are of high quality, this card will be a lot better.
If anyone remember the initial reaction to Lyra the Sun Shard, it was pretty similar seeing as most classes have a lackluster spell line-up when pulling random cards out of it (Low value Paladin spells, Freeze support Mage spells, the overcosted Warlock Spells), but that card became a stable of Priest decks for that format and is still seeing pretty consistent play at the moment.
The Battlecry is worth between 4 and 5 mana by itself too, and with that and the Hero Power, I think some sort of anti-aggro if not full on Control Shaman will become possible perhaps on the back of the elemental package with Volcanos, Healing Rains, and this. Granted, such a situation is kinda scary for anyone who lived through the dominance of Midrange Shaman about two years ago now seeing as the class will still have Flametongue Totem and Bloodlust.

This card, thankfully, is not a good fit what I imagine Control and Cube Warlock decks will look like in April when the rotation takes place.

However, this does seems to be a pretty decent addition to Zoo Warlock, which will be very much appreciated since that archetype is losing critical cards such as Darkshire Councilman and Melchezaar’s Imp. The Taunt minions Warlock has access to would increase the chances of Glinda surviving an on curve play.

I think this will be Warlock’s version of Fandrel Staghelm, a card you either kill the turn it is played or lose to. Granted, Fandrel is much easier to combo with (Turn 6 Fandrel + Power of the Wild) and easier to slot into both the control and aggressive archetypes.

Most likely not. However, it cannot be said for sure until play testing can be done.

If you were to play Glinda after Skull of the Man’ri summoned a Voidlord, it never had the Echo Keyword in your hand. This is the only interaction we know for sure.

If you were to have Glinda on the board when Skull summoned a demon, a copy of it still should not be added to your hand. This is because the current description of Echo is that another copy of the card is added to your hand when it is the card with Echo is PLAYED not just summoned as Skull currently does for the demons it pulls out of your hand.

I doubt that particular combo will see play seeing as it is a 2 card, 10 mana combo.
While the 7 health will allow it to live on the board on curve quite often, throwing it out on turn 6 unless its placed behind a Voidlord or at least one other taunt won’t usually end up going well. Granted, this card does not fit into Control Warlock as we currently know it unless filling your board with Gnomeferatus is that appealing.
Similar to Fandrel, It will be best to combo this with something the turn it is played, and on turn 10, throwing out this and 4 void walkers seems pretty decent for a Zoo deck.