To kick this off here is the optional features table from spec section 21 with a column added for IBM DFDL

Feature	Detection	IBM DFDL
Validation	External switch	Yes
Named Formats	dfdl:defineFormat or dfdl:ref	Yes
Choices	xs:choice in xsd	Yes
Arrays where size not known in advance	dfdl:occursCountKind 'implicit', 'parsed', 'stopValue'	Partial - not 'stopValue'
Expressions	Use of a DFDL expression in any property value	Yes
End of parent	dfdl:lengthKind "endOfParent"	No
Simple type restrictions	xs:simpleType in xsd	Partial - not unions
Text representation for types other than String	dfdl:representation "text" for Number, Calendar or Boolean types	Yes
Delimiters	dfdl:separator <> "" or dfdl:initiator <> "" or dfdl:terminator <> "" or dfdl:lengthKind "delimited"	Yes
Nils	XSDL nillable 'true' in xsd	Yes
Defaults	XSDL default or fixed in xsd	Partial - unparsing only
Bi-Directional text.	dfdl:textBiDi 'yes'	No
Lengths in Bits	dfdl:alignmentUnits 'bits' or dfdl:lengthUnits 'bits'	Yes
Delimited lengths and representation binary element	dfdl:representation 'binary' (or implied binary) and dfdl:lengthKind 'delimited'	Yes
Regular expressions	dfdl:lengthKind 'pattern', dfdl:assert with dfdl:testkind 'pattern' , dfdl:discriminator with dfdl:testkind 'pattern'	Partial - only for dfdl:lengthKind
Zoned numbers	dfdl:textNumberRep 'zoned'	Yes
IBM 390 packed numbers	dfdl:binaryNumberRep 'packed'	Yes
IBM 390 packed calendars	dfdl:binaryCalendarRep 'packed'	No
IBM 390 floats	dfdl:binaryFloatRep 'ibm390Hex'	Yes
Unordered sequences	dfdl:sequenceKind 'unordered'	Yes
Floating elements	dfdl:floating 'yes'	No
dfdl functions in expression language	dfdl:functions in expression	No
Hidden groups	dfdl:hiddenGroupRef <> ''	No
Calculated values	dfdl:inputValueCalc <> '' or dfdl:outputValueCalc <> ''	No
Escape schemes	dfd:defineEscapeScheme in xsd	Yes
Extended encodings	Any dfdl:encoding value beyond the core list	Yes
Asserts	dfdl:assert in xsd	Partial - not on global elements or simple types
Discriminators	dfdl:discriminator in xsd	Partial - not on global elements or simple types
Prefixed lengths	dfdl:lengthKind 'prefixed'	Partial - not nested prefixed lengths
Variables	dfdl:defineVariable, dfdl:newVariableInstances, dfdl:setVariable Variables in DFDL expression language Note that variables as a feature is dependent on the Expressions feature.	Partial - not newVariableinstance or external variables
BCD calendars	dfdl:binaryCalendarRep "bcd"	Yes
BCD numbers	dfdl:binaryNumberRep "bcd"	Yes
Multiple schemas	xs:include or xs:import in xsd	Yes
IBM 4690 packed numbers	dfdl:binaryNumberRep "ibm4690Packed"	Yes
IBM 4690 packed calendars	dfdl:binaryCalendarRep "ibm4690Packed"	Yes
DFDL Byte Value Entities	Use of %#r syntax in a DFDL String Literal other than the dfdl:fillByte property	Yes
DFDL Standard Character Set Encodings	dfdl:encoding name begins with "X-DFDL-".	No
Bit Order - Least Significant Bit First	dfdl:bitOrder with value 'leastSignificantBitFirst'	No

Some IBM DFDL restrictions are not captured in the table, mainly due to non-implementation of many of the later errata. Full details here https://www.ibm.com/support/knowledgecenter/SSMKHH_10.0.0/com.ibm.etools.mft.doc/df00150_.htm but the key omissions are:

• fn:count(), fn:exactlyOne(), fn:name(), fn:local-name(), fn:namespace-uri()
• BOMs
• Correct distinction between 'empty' and 'missing' when parsing (action 140)
• Use of namespaces in expression paths
• dfdl:separatorSuppressionPolicy "trailingEmptyStrict"