Hi,
the problem with your code is that sayGood is actually a value and not a function. It is evaluated when it is declared (even before the taskBody2 task is created). The value assigned to "sayGood" is unit (because that's what the printfn function returns). The following code should clarify the problem:

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let test = 
   printfn "initializing 'test'"
   42
printfn "value of test: %d" test

Note that the "initializing..." string is printed before the "value of..." (because the value of "test") needs to be initialized during the let-binding. To correct this, you need to declare "sayGood" as a function (taking for example unit as an argument). Then you can call it from the lambda function passed as a parameter to task..

Also note that the WaitAll method takes a param-array of tasks that you want to wait for as a parameter (AFAIK calling it without parameters doesn't do anything).

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// function taking string and returning unit
let sayHello englishText = 
    printfn "%s" englishText
    Thread.Sleep(10000)
    printfn "Again: %s" englishText

// a function taking unit and returning unit
let sayGood () = // note: added "()"
    printfn "I am fine and you?"
    Thread.Sleep(10000)
    printfn "Again: you?"

// Note: we can pass lambda functions directly to StartNew
let t1 = Task.Factory.StartNew(fun () -> sayHello "Hello, World!")
let t2 = Task.Factory.StartNew(fun () -> sayGood ()) // pass unit as an argument here

Task.WaitAll(t1, t2);;