Novice Engineer. One mana drops were rare outside of aggro decks.
Novice Engineer had become ubiquitous. So much so, that the early game had become predictable. You knew exactly what to expect from almost every deck you played against in the early turns.
Since the nerf to Novice Engineer the early game has become far less predictable.
I see far more 1-mana drops now. I see far fewer Novice Engineers, to the point that when I do see one I feel actual surprise. People are adding necessary card draw to their decks in more challenging ways.
Loot Hoarder has not become the de facto Novice Engineer replacement that most thought it would. I don't see Loot Hoarder any more than I did pre-January 16th. And I might actually be seeing it less.
I'm seeing Argent Squires more frequently on turn one. Same with Leper Gnomes, Worgen Infiltrators and Southsea Deckhands. Turn twos are filled with more Mad Bombers and Bluegill Warriors. I'm not saying these cards were not played before the Novice Engineer nerf, but I'm definitely seeing them more often now.
Acolyte of Pain has seen a bump in popularity. The same for Cult Master. You're seeing people experiment more with Coldlight Oracle. And among the legendaries, while already popular pre-nerf, Bloodmage Thalnos and Nat Pagle have become a tad more popular.
The nerf did exactly what Blizzard hoped it would do. It created far more variety in the early game, and changed up some of the mid-game, thus adding more unpredictability to matches. The nerf to Novice Engineer has made Hearthstone a better game.