I could list some of the reasons Rob Buckley gave for why it was just a great episode of classic Who-ness, but in terms of the 'phwoar' factor it had to be glorious ease with which Tennant both suitably adopted and then slid out of using his Scottish voice.
I mean, I do understand completely why they didn't go with him using his native accent throughout the series, but there is just something lush about his speech patterns. And it was SOOO well done how he switched from one to another.
And next week of course we have 'Giles', Sarh-Jane Smith AND K-9. Almost makes it worth having to strip a room this weekend ready for the new bathroom being built (it will be sometime before installation!)