Yeah Ming Na is definitely going to give the show a bit of "cred" and she does look amazing for a woman her age - although in some ways it really depends on how she's shot. A bad angle or the wrong lighting and she looks like she could be playing the "Grandmother" character and another angle and she looks younger than she was during her ER days. Although she always did have that kind of face, even during Joy Luck Club - she was supposed to be so young and yet sometimes when they shot her, she looked a lot older and it wasn't due to any makeup change. Good angles and bad.
But anyway - I think it IS cool that they cast someone "out of the demo" to be the ass kicker of the show. They didn't make the young blonde one the super duper martial artist - as per most of Joss Whedon's shows - so it was a nice change to the plan. Plus one upshot is - it's been quite a while since ABC had a prime time spy show on the network and I think really SHIELD could fit in very well next to Castle as the procedural cop show with quirky characters but they're spies. I still expect next week's Castle to reference SHIELD in some way.
MARK MY WORDS. THERE WILL BE A SHIELD REFERENCE ON CASTLE!