More syllables makes it more reliable I think. Slang/shorthand kinda works now, so 'I wanna play Madden' will play Madden NFL 16 (or ask you which Madden and present you with options - but I haven't experienced that yet) or 'play Guardians of the Galaxy' instead of 'play Guardians of the Galaxy plus bonus features'.
You can see what it's heard, and adjust what you said if it's wrong, previously it would just kinda think and then fail leading to you just saying it again (though usually louder).
It's a more conversational experience, rather than the strict commands. The tradeoff is speed a little bit, but I'm not sure. My wife pauses for Cortana to recognize her after the wakeword, but that's led to her then saying the command (like Pause) while Cortana is thinking about what she said, which since it was just the wakeword Cortana says 'that's me!', lol. Then her hair gets redder and she glares at me. Just saying 'Hey Cortana, pause' is sufficient, but feels like maybe it takes a tick longer than the normal Xbox command.
It's under heavy development still, things will get better, pretty happy thus far though.