Some commands, I guess when the command is supposed to be run be the client itself,
they won't work since the "execute" command is just "sudo-ing" the player.

The affected commands are:
"/help" , "/seed" , "/spawnpont" , "testforblock" , "tell" ... and so on

For example: Put this command in a commandblock and run it.

execute @p ~ ~ ~ /tell "playername" hello

What I expect:
When the command is run, it should force the nearest player to write a message to "playername" (here you write either @p(c=-1) or a specific name)

What actually happens:

• Nothing -
  There is no error message in commandblock, neither the target player is receiving a message, nor is there a error displayed to the sending player.

When you run these commands by yourself, you will get a output in you chat.
With "/help" you will get the list of commands, with "/seed" it will show you the seed code, "/spawnpoint", "/tell" will write a message to a player.

What they all have in common is that they give a output in the chat.

Another example:

execute @p ~ ~ ~ /help

What I expect:
It should show the help menu to the target player

What actually happens:

• nothing - , nothing is shown in the chat, also no errors.

If you use the tellraw funktion with clickEvent + runCommand and use there "/help" as the command, it will work. It shows the help menu.
Also with books this works. for example:

/give @p minecraft:written_book 1 0 { title:"", author:"", pages:[ "{ text:\"Click.\", clickEvent:

{ action:run_command, value:\"/help\" }

}" ]}

So my suggested fix is, that when you run the "execute" command, it works the same way as "tellraw" with "runCommand"