Sarah Jane, like Rose, traveled with the Doctor from one regeneration to the next and was quite chummy with the ol' timelord. With the possible exception of Romana, Sarah Jane was probably the Doctor's closest equal in both brains and moxie. So hurrah for a return appearance!
PS: Yes, I know I could have watched the whole episode on YouTube by now -- but I wanna see it on TV, dangit!
PPS: This is also assuming that YouTube works. It's been acting wonky at me all day!