I haven't seen the episodes surrounding Sarah Jane Smith's (Elisabeth Sladen) parting with the Doctor, but I have seen other abrupt partings that the show never really dealt with to my satisfaction; 'til now, that is. Their reunion was very believable - as was Rose's (Billie Piper) reaction - especially Sarah Jane's horror at learning that the Doctor'd never mentioned her to Rose.
K9 (voice of John Leeson) was funny too, although, again, I haven't seen many of the later Tom Baker era episodes with him; makes me wish all the more that BBC Kids hadn't skipped over his years in their recent run through most of the Doctor Who episodes; must've been a copyright issue.
Oh, and a final thought on David Tennant: Not since Jon Pertwee have I really felt the Doctor's age - in his world-weary sighs in Pertwee's case, but more in his eyes, and what he doesn't say in Tennant's case; his
I used to have so much mercyline was just brilliant.