Boss Rush RUSH SURGE: a free upgrade pick every time a boss drops to 75%, 50% and 25% HP, so every fight speeds up as it goes
Boss Rush BONUS ROUNDS: after every boss, a 12 second minigame for extra coins that doesn't count toward your time and can't hurt you: Pinata Panic (smash a giant golden pinata), Coin Shower (grab the raining coins before they fade) or Golden Stampede (catch five golden potatoes for coins and free crates)
Balance
Boss Rush is much faster: bosses have far less HP (12k up to 170k, was 30k up to 649k), so each fight takes about a minute instead of several
Cupcake's phase 2 dodge gauntlet is half as long: 15 seconds, was 30 (phase 3 is unchanged)
Co-op is easier, especially with 2 players: each extra player now brings 90% of an extra horde (was a full one), +15% enemy HP (was +25%), +5% damage taken (was +10%) and +100% boss HP (was +115%)
Co-op elites keep their normal HP instead of getting the extra-player HP; you still get an extra elite for each extra player, but they now arrive one at a time, 9 seconds apart
In co-op there's never a second Elite Axe Knight on the field while one is still alive, so one dash always gets you through the slams
Co-op revives are stronger: back up with 35% HP with 2 players, 30% with 3 and 25% with 4 (was 30%, 25% and 20%)
The level up screen says when a pick is a RUSH SURGE and how low the boss is
The wave banner shows which bonus round you're in
The co-op lobby room's difficulty line shows the new numbers and that elites come one at a time
Boss Rush summons had far too much HP, some more than their own boss: Cupcake's demon dogs, the Garlic Titan's brutes, the Demonic Lord's demons and imps and the Broodmother's spiderlings now have HP in proportion to their boss (Cupcake's dogs: about 375 HP, was about 11,000; the Demonic Lord's demons: about 4,300, was about 296,000)
Co-op hasn't had a big online test yet, so expect some rough edges
You can't join a co-op run that has already started, and if the host leaves the game ends for everyone
Weapon fire rate is very slightly tied to frame rate
The Elite Hunter achievement description on the achievements page may still show the old text