First, let's compare the old heating system with the new one.

With the old heating system, the forge didn't exist and fire crystals were not necessary for leveling up an item. You could skip directly to the next heat level on a piece of equipment without forging said equipment. However, mist energy was necessary to operate elevators and a player was only issued a limited amount of the stuff per day, therefore a player could only collect so much heat in one day.

With the new heating system, some fire crystals ( a handful of crystals for 1* gear, a few wheelbarrows of crystals for 2* gear, a pickup truck full of crystals for 3* gear, a convoy of semi trucks full of crystals for 4* gear, and a Batillus-class oil supertanker full of crystals for 5* gear) and a trip to the forge are necessary to level up a piece of gear once it's heat bar for the current level is full. However, mist energy no longer exists and taking a trip down an elevator in the Clockworks is a free action.

There's a bit more to forging mechanics than dumping some fire crystals and the desired piece of gear into the Forge, waiting a few seconds, and retrieving you now heavily-charred equipment* with a pair of asbestos gauntlets.**
Let's take a closer look at the three options available for leveling up your gear.

20%: You'll almost certainly waste more crystals by forging at 20% than you would save by not just forging at 100% power. Generally a bad idea, but if you offer enough sacrifices to the RNG Deity, who knows?***

75% / 70%: An insidious scheme to get more money. If you fail this even once, having to forge again will require almost 1.5 times as more crystals as the 100% option, and you might not even get the desired results on the next try.

100%: Grind grind grind grind grind, and maybe grind some more. You can try offering some more sacrifices to the RNG Deity**** and ask for fire crystals to drop more often, but it is still bound to be an extremely tedious task.

Let's take a closer look at option 2#; the 75%/70% one. With the previous heating system, you were bound to eventually get all level 10 gear as long as you were semi-active and didn't waste you mist on rage crafting too often. However, with the new heating system, you could theoretically grind fire crystals and repeatedly fail forever if you only used the 70%/75% option, as the RNG Deity (otherwise known as probability) gives no fraks to the affairs of mere mortals.

Old system: Buy lots of mist energy to get heat in the Clockworks and get a guaranteed level up.
It is about the same cost as:
New system: Buy fire crystals, but only enough for the 70%/75% value.

TL;DR: New heating system actually makes the game harder.

*I personally believe the Blackened Crest was a result of testing the prototype forge with some unlucky shield "borrowed" from a player's arsenal. The story behind Almire was just a cover-up, and Almire itself (and its undead inhabitants) are simply extremely advanced animatronics. The dev team personally hacked each Knight's personal Heads Up Display Unit to ensure the animatronics would appear on the HUD as undead/fiends and not constructs.

**Mysteriously appeared in each Knight's arsenal after the 2013-07-30 patch. Where they came from, or how they got into the arsenals is a complete mystery.

*** I hear s/he prefers Snarbolaxes roasted over shadow fire.

**** Just to clarify, the RNG Deity is most certainly, absolutely, not Vog....great, a mob of disgruntled forge-failees are now hunting around depth 25 with Final Flourishes and Wild Hunting Blades. And just as we were about to get some more intel on the Core, too.