exoteric said:

I'll start out -


Tuple Syntax 
a la (precise parenthesization not so important here)


Tuple Enumeration

Not a language feature but allow tuple to be enumerated (IEnumerable<T>) for both homogeneous and heterogeneous tuples, meaning these two cases would both be legal

foreach (var x in (1,2,3))
foreach (var x in (1,"WTF",3))

Where in the first case the tuple would be IEnumerable<int> and in the second case the tuple would be either IEnumerable<IDynamic> or IEnumerable<Object>.

BigInt literals


var bigint = 349438474346346348348346734634634892390139328234746546348349349347237236465483932948565623893832932932737;