Interaction with other language features:

General concerns:

• Error handling/recovery
  • Errors in parsing
  • Error handling for semantic errors (e.g. ambiguous lookup, inaccessible lookup, wrong kind of thing found, instance vs static thing found, wrong type for the context, value vs variable)
• Public interface of compiler APIs, backcompat scenarios
• Determinism
• Atomicity
• Edit-and-continue
• Completeness of the specification as a guide for testing (e.g. is the spec complete enough to suggest what the compiler should do in each scenario?)
• Other external documentation
• Performance

Types and members:

• Access modifiers (public, protected, internal, protected internal, private), static modifier
• Parameter modifiers (ref, out, params)
• Attributes (including security attribute)
• Generics (type arguments, constraints, variance)
• default value
• partial classes
• literals
• enum (implicit vs. explicit underlying type)
• expression trees
• Iterators
• Initializers (object, collection, dictionary)
• array (single- or multi-dimensional, jagged, initilalizer)
• Expression-bodied methods/properties/...
• Extension methods
• Partial method
• Named and optional parameters
• String interpolation
• Properties (read-write, read-only, write-only, auto-property, expression-bodied)
• Interfaces (implicit vs. explicit interface member implementation)
• delegates
• Multi-declaration

Code:

• Operators (see Eric's list)
  • Operator overloading
• Lvalues: the synthesized fields are mutable
  • Ref / out parameters
  • Compound operators (+=, /=, etc ..)
  • Assignment exprs
• lambdas (capture of parameters or locals, target typing)
• execution order
• Target typing (var, lambdas, integrals)
• Type inference
• Conversions
  • Implicit (identity, implicit numeric, implicit enumeration, implicit nullable, null literal, implicit reference, boxing, implicit dynamic, implicit constant, user-defined implicit conversion, anonymous function, method group)
  • Explicit (numeric, enumeration, nullable, reference, unboxing, dynamic, user-defined)
  • Anonymous functions
• nullable (wrapping, unwrapping)
• OHI
  • inheritance (virtual, override, abstract, new)
  • overload resolution
• Anonymous types
• Unsafe code
• LINQ
• constructors, properties, indexers, events, operators, and destructors.
• Async
• Var

Misc:

• reserved keywords (sometimes contextual)
• pre-processing directives
• COM interop