There have been many articles and blogs about Leonard Slatkin at the MET, including this one by Anne Midgette. Is TRAVIATA hard to conduct? Mostly, the answer is no. But the orchestration of the show is very naked and faulty coordination shows up very easily. Things like the famous "Libiamo" have some tricky breaths that must be agreed upon and the worst is Alfredo's second act aria, "De' miei bollenti spiriti." Coordinating any kind of tempo variance with strings playing pizzicato is difficult, but at the pace of sixteenth notes as it is in this aria, it really requires some finesse, rehearsal, and a sense of agreement about what is going to happen.
I remember early in my career a production of COSI FAN TUTTE at Florida Grand Opera where a group of very well known professionals ate a director alive. He was an experienced director, but instead of coming in with a clear vision, he simply was intending to see what the cast was used to doing. Given all the hard work it takes to prepare a role--the hours and hours it takes to learn it, memorize it, shape it--the cast didn't appreciate it at all.