I didn't get around to watching the episode until today... I've been disinclined.
Nonetheless, it was an average episode, the positives outweighed the negatives, and I happily gave it a 3 on OG.
The Spinal Tap reference... yes, it's obscure enough that not everyone will get it, it's 23 years old now and some of the lingo has permeated our language quite effectively, and I didn't hate it. I like Warren's comment about how such things take away from the drama and action for a cheap gag, but it's not the first time it's been done in the series (old or new) so I'm okay with it. I happen to love This Is Spinal Tap, so I may also be biased.
As for the "reverse the polarity" throwback, I loved it. I love throwbacks more than I hate them and Tennant saying the line just felt natural.
Rank: Third best of the series behind Gridlock and ahead of Evolution of the Daleks. Yes, Evolution outranks The Shakespeare Code in my world. =)