If all of the Earthdawn Fourth Edition (ED4) sub-systems were to have some yearbook style votes for their class, spellcasting would be voted most changed. It isn't complete different from where it started by any means - the basics are still the same. However, spellcasting and spells have received the most thorough redevelopment.
The degree of changes made run the spectrum. For example, Read and Write Magic was exchanged for Patterncraft to better embrace the various functions the talent has had previously. This also gives a chance to hang new functions on it in the future. The use of a grimoire has been updated: when casting from your own attuned grimoire, you gain an extra success. A minor benefit for use outside of combat; the goal was to give more life to an item which is supposed to be central to a character, but was seemed to be ignored unless a new spell was being added. Also, each success on a Thread Weaving test weaves a thread for a spell.
A more significant overhaul is how illusions function. They have been divided into two categories: figments and true illusions (which are simply referred to as illusions). The distinction is (hopefully simple): a true illusion contains a powerful compulsion to behave in a particular fashion, while a figment is a trick on the senses. Interacting with a figment will immediately reveal the truth, while interacting with an illusion is considerably more difficult since its very function convinces you it is real. The primary mode to detect an illusion is through a sensing test, which is essentially any interaction with the illusion through which you may learn the truth. The greater the evidence something is wrong, the less hold the illusion has over you. Which means if all of your friends have seen through the illusion, the need to lasso you to haul you out of an imaginary box should be significantly reduced.
Illusions use for combat turn their compulsion into a psychic assault on the target, though only if the target attempts to resist the effects, buying into the illusion. If you disbelieve the illusion, it simply washes over you. However, if you disbelieve a spell which isn't an illusion, it can get ugly. There isn't a roll for disbelief, just a simple choice if you think it the fireball coming at you may be an illusion.
The biggest overhaul has been spells themselves. Every spell included in ED4, at least ten first circle spells and five for each other circle, was redeveloped. Each spellcasting discipline also received a number of new spells. Thread weaving difficulties (circle + 4) and reattuning-on-the-fly difficulties (circle + 9) were standardized, spells now have effects for extra successes, and extra threads for each spell were included.
Extra threads represent a dynamic shift in how spellcasting works. A spellcaster can weave a number of additional threads based on their circle in the appropriate discipline. For example, a first circle Elementalist can weave one extra thread, while a fifth circle Elementalist can weave two extra threads. This allows a spellcasters to devote more time to a spell's pattern in exchange for tailoring the spell to their needs. Outside of an action sequence, when they have all of the time they need, this means spells can be effectively maximized and will scale up as circle increases. In combat, a 0 thread low circle spell can still have life by being placed in an enhanced matrix and pre-weaving an extra thread for a specific function each time it is cast. For example, Earth Darts could be placed in an enhanced matrix for an additional target, improve damage, improve the range, improve its armor reduction effect, or increase the duration of its armor reduction effect.
The number of combat spells which require numerous threads was also scaled back considerably. This is particularly true for low circle spells; higher circle spells tend to have more multi-thread spells, though this is reflected in their significantly larger effects. As every spellcaster knows, it is hard to justify big spells all of the time when the circumstances may be completely different by the time they are actually cast. Essentially, low circle spells now fill the general "bread and butter" style spells which should remain useful throughout an adept's career, rather than continually being cast aside for something bigger and better.
During the redevelopment, particular care was given to ensure spells reflected the core themes of each discipline. Each of the disciplines has particular things which they are good at, and those which they may not be able to do at all. For example, all Elementalist spells are elemental and physical in nature, Illusionist spells have a strong deception streak, but also outright break the rules, Nethermancer spells commonly traffic in fear and spirits, featuring numerous debuffs, and Wizard spells reflect their versatility and mastery of pure magic.
Future essays on each discipline and their spell list will discuss what they do well.