CK2Plus (CK2+) is a mod created largely by Wiz, a member of the Something Awful forums who began working for Paradox in January 2013. The mod also incorporates elements from other mods on the Paradox forums, as well as work done by other users on Something Awful's Paradox thread. In Wiz's own words:
The purpose of CK2+ is to create a broader, deeper, more challenging and more balanced CK2 experience, without straying too far from the original game mechanics or tacking on deterministic railroading events and modifiers. The purpose of this mod is not historical accuracy (although I try to preserve it whenever it is not a major detriment to gameplay) but rather to embrace and enrich the medieval sandbox intrigue simulator that is CK2, while fixing its various little issues and exploits.
CK2+, as Wiz says, aims to deepen the CK2 experience, both in detail and enjoyment. Due to changes in mechanics that may make the game more difficult for new players, it is recommended to have played the vanilla game enough to have a good understanding of its mechanics.
 Changes to CK2
- Tweaking of vanilla education mechanics to ensure wards don't always end up with their tutor's stats.
- A new tyranny system similar to EU:Rome. Kings cannot wantonly revoke titles, imprison, banish or execute subjects any more without incurring Tyranny. The "Tyrant" trait lowers relationships with everyone and in turn makes it more likely for vassals to revolt.
- Crown authority is now more difficult to raise (requires a certain prestige score), but brings greater rewards.
- Changing of Crusade/Jihad mechanics to more accurately and fairly represent large-scale holy wars. Winning a crusade/jihad results in the de jure kingdom of the war's target being created (e.g.: a successful Crusade for Jerusalem results in the Kingdom of Jerusalem being created and awarded to the victor), as well as the institution of a special truce to help Crusader states survive more than a few years.
- Holy wars require piety to fight (this is to avoid the spamming of holy wars that happened in Vanilla). Religions with a religious head can petition for permission to fight a holy war, which removes this piety penalty (available in the decisions box of the intrigue window). Permission for holy wars are granted based on piety and relationship with the religious head.
- Adds sub-Saharan kingdoms of West Africa, including Songhai, Mali and Sokoto (n.b.: a similar feature is now present in vanilla CK2 as well).
- Adds a greater list of ambitions and plots, including: "Get a lover", "Break out of prison", "convert a province", "win a war", etc.
- More cultures, as well as cultures to represent "going native": Normans become Sicilians or English, Saxons become Anglo-Saxon, Norwegians become Anglo-Norwegian, Crusader states become Levantine.
- It is now generally harder to convert province religion/culture.
- Civil wars become more dangerous: when a vassal declares war on their liege, they ask other vassals for support. A king can end up at war with most of their kingdom if they are unpopular among their dukes.
- Empire titles can be created by holding four kingdoms and an appropriate capital province, and are often limited to specific culture groups (e.g.: the Spanish empire requires holding Toledo and being Castillian, Catalan, Portuguese or Basque).
One of the first things you'll notice playing this mod is that demesne limits have been drastically reduced, and legalism as well has been halved in effectiveness. As a count you may be quite normally limited to 2 or 3 holdings - the majority of limit increase will come with a rise in rank as opposed to stewardship or legalism. This has the effect of making you much more reliant on your vassals as your personal holdings will never be particularly large.
Another major thing you'll notice, though probably not pleasantly at first, is that when your vassals rebel they automatically make a call to every other vassal and if they like them better than you, regardless of your personal relationship, they will honor that call. This can have a very drastic effect making a minor count rebellion go from a minor nuisance to a dangerous uprising of half your vassals.
De jure wars against other religions have also been disabled, making any territory gains against other religions have to be holy wars, which make the automatic call. If somehow you have another CB against another religion, all of them now make a call to their neighbors similar to a holy war.
In general, quite a few of the changes lead to gameplay being more restrictive on your personal power especially regarding acquiring new lands which, depending on your personal playstyle, can either be a bad thing or lead to a deeper, more involved vassal-reliant game.
CK2Plus also adds a lot of mini-mods, such as avee's minimod for family relations and the West Africa expansion along with new ambitions and others.