Mathematica 12 both significantly extends the capabilities of Mathematica and introduces many enhancements that make the system even smoother and more efficient.
Automatic generation of integration, summation, etc. constants
Multivariate limits, lim sup, sequence limits and all that
AddSides, MultiplySides, DivideSides, etc. (but always correct)
Big O, little o, WKB, asymptotic solutions and all that
Generate symbolic proofs from axioms with FindEquationalProof
Complex plane, complex values; 2D, 3D; functions, data
Simpler formatting for Root[] objects
etc. to indicate formal variables more elegantly
Give your own scaling function (log, probability, etc.)
Show multiple datasets in stacked, percentile, etc. form
Arrays of plots with common axes
Full computational treatment of polyhedra (served beveled, snubbed, ...)
Polygons that wind, cross, have holes, etc.
Automatically visualized regions (implicit, explicit, ...)
Euclid-style geometry (CircleThrough, Midpoint, ...) solved with FindGeometricConjectures etc.
x ± dx for computation, visualization, etc.
SystemModel, SystemModelSimulate, SystemModelPlot, ... for machines, circuits, pathways
Quadratic, semidefinite, conic, ... optimization for problems big and small
Visualization and computation with arbitrary (organic) molecules
Read, write, analyze, transact on Bitcoin, Ethereum, etc.
Now there is a single function to sort largest first
Properties are automatically inherited when graph operations are done
Standardized way to get information on objects (images, audio, cloud objects, entities, ...)
Specify a day, week, month, etc. as well as a moment in time
Association-based specifications for cellular automaton rules
In[•] and Out[•] to label input and output, even without a kernel
Store code and data directly in a notebook, formatting only as an icon
Summaries of Audio, SparseArray, etc. give a choice of storage location
New Presenter Notebooks for rich screen size–adaptive interactive presentations
Full support of modern 64-bit Cocoa for notebooks on the Mac (> 2 GB etc.)
Automatic indicators to make it easier to notice closed cell groups
Qt 5 support, enabling rendering without an X server
GeoBubbleChart to visualize values on maps
RandomGeoPosition for random points in any region (country etc.)
AttentionLayer etc. integrated into the symbolic neural net framework
Automatic anomaly detection for all forms of data
FeatureSpacePlot, FeatureSpacePlot3D... and lots of built-in feature extractors
State-of-the-art, high-efficiency neural net training
Fully interactive machine learning dashboard for training and more
Automatic visualization of neural net structures (with drill down)
MXNet export of core neural net structures
The Wolfram Neural Net Repository with immediately computable nets for NetModel
LearnDistribution for distribution of any type of data
FacialFeatures for finding faces and their features in an image
ImageRestyle for automatic restyling of images
ImageGraphics to heuristically convert bitmaps to vector graphics
Capture sound to an Audio[] object
Infer audio from a spectrogram image with InverseSpectrogram
M4A import and export
Transcribe text from speech audio with SpeechRecognize
Synthesize speech from text as computable audio
State-of-the-art Q&A from textual passages with FindTextualAnswer
Countries, dates, movies, whatever: pick them out of text with TextCases
Translate between languages using an external service
Experimental compiler from the Wolfram Language to LLVM
NumericArray for efficient storage of raw types (Integer16, ...)
Reduced download footprint
Automatic predictive (or manual) prefetching of Wolfram Knowledgebase data
First, largest, most, etc. direct queries on entity types and classes
Find pages and images with WebSearch and WebImageSearch
Programmatically settable initialization of values
Create and verify digital signatures
Create and edit .wl and .wls files from File > New
In a session, on your machine or in the cloud
Import, export, string-convert, etc. directly with byte arrays
This notebook, this machine, this user, ...: persist code and data anywhere
Asynchronously submit to a URL, with callbacks
MemoryAvailable for programmatic access to memory limits
Use RemoteFile in CopyFile etc., authenticating as you need
Connect to any IMAP server and read/analyze/modify mail folders
RemoteRun etc. to run processes through SSH on remote machines
Programmatic access to ps-like data on running processes
Capture network packets going to and from your machine
Read and write data in MongoDB
Full symbolic layer for seamlessly accessing SQL databases
Programmatically access graph databases and triplestores
Much more efficient (and robust) import and export of tabular data
WebImage to get an image of any webpage
Complete import and export of expressions to HDF5
Real-time and asynchronous financial data feeds (Wolfram Finance Platform only)
Deploy control systems and filters directly to microcontrollers
Use > to start a Python cell, then get results back in the Wolfram Language
Programmatically interact with a web browser
ExternalEvaluate to call code directly from the Wolfram Language
OAuth etc. to authenticate access to external services
Create geometry, get telemetry... or programmatically make a full game
Instantly access (and contribute to) a public repository of functions
File > Publish to Cloud: for the world, or with a password
Base URLs now allow usernames in addition to UUIDs