Common things that are known to have problems or not work in Rakudo as of 2009-01-02.
(Where appropriate, provide a reference to the corresponding RT ticket.)

• arrays in list declarations -- e.g., my ($b, @a) = ...
• arrays and hashes as parameters in methods/subs
• reading array/hash elements causes them to autovivify (RT #61882)
• incorrect parsing of blocks followed by 'if', 'unless', 'while', 'until', 'given', etc. (RT #57876)
• accessing outer lexicals from eval
• lazy lists
• lazy ranges
• prefix and postfix hyperoperators
• //= (RT #61880)
• operator overloading, defining new operators
• nested package/grammar/class declarations
• autothreading of methods over Junctions
• slices/indexing of Ranges (RT #61848)
• filling positional parameters by named argument (RT #54808)
• assigning values to %*ENV (RT #57400)
• methods on Match cannot be used as rule names (RT #57864)
• some things that should be readonly aren't

Things that do (mostly?) work in Rakudo:

• scalar, array, hash variables
• selected infix hyperoperators, reduction operators, cross operators
• slicing syntax
• whatever star in slices
• closures