Andris Nelsons has conducted only one complete concert at Tanglewood, a Stravinsky-Brahms program last summer, and the impression he left was: The blood runs hot.

Though the temptation was to say, "O.K., take a breath, it'll be all right," you can't know a conductor on first date. So doubts go on the shelf and hopes rise to the top as, with the announcement last week, Nelsons is set to become the Boston Symphony Orchestra's 15th music director.

The contrast with his predecessor, James Levine, could not be greater:

Levine, then 61, came to the BSO as a veteran maestro equally committed to another prime American institution, the Metropolitan Opera. His conducting bore the touch of an old master. The Latvian Nelsons, 34, comes as a young man known primarily as a guest conductor in Europe. Critics seem to agree: Energetic. Kinetic. Fiery.

Levine, though an inspired and inspiring choice, was felled after a few years by a spectrum of physical problems. Nelsons presumably comes with a clean bill of health, or after the Levine experience he would not have been hired.

Levine retained New York as his base. Nelsons has spoken of making Boston his home and mixing in the city's life, including attendance at sports events.

The two-year wait for a new music director appears to have paid off. The public -- insofar as there is a public for classical music anymore -- may have grown impatient, but the BSO surveyed the field, tested the candidates in concert and chose with care.

It's worth recalling that the hopes and rejoicing for Nelsons are the same as they were for Levine. But it's to Seiji Ozawa that you really have to look for a benchmark. Ozawa (also a sports fan) came to the BSO in 1973 at age 38 as a young fireball.

By the end of his 29-year reign, Ozawa, sadly, had fizzled out (as, sadly, did Levine). That proves nothing about Nelsons, of course. But it does mean that the director-orchestra relationship is a complex one that depends on long-term compatibility and growth.

The BSO appears to have avoided the problem of divided loyalties in signing Nelsons to a five-year contract, beginning in 2014. His term as director of the City of Birmingham Symphony Orchestra in England has two years to run, after which, according to the announced BSO contract, he will be free elsewhere to be a principal conductor, which involves few administrative duties, but not a music director.

Nelsons has performed regularly, if not frequently, with the BSO over the past two years, sometimes as a stand-in for Levine, sometimes in his own right. From all reports, the players like him.

Principal hornist James Sommerville, a member of the search committee, told The Boston Globe the challenges brought on by the Levine era played a part in the 12-member committee's thinking.

"I think it's fair to say that we, as an institution, wanted someone who would bring a lot of vitality, physical vitality to the position, and someone who we could really feel was going to really take on the role of being a Bostonian," he said. "To really be here."

The big blank spot -- especially big for the Berkshires -- is Nelsons' participation at Tanglewood. He will conduct eight to 10 weeks in Boston during 2014-15 and 12 weeks -- about average for a music director these days -- in following seasons, but his Tanglewood commitment remains undecided. Meanwhile, he works during summers at the Wagner festival in Bayreuth, Germany.

In Levine's early Tanglewood years, he brought energy and excitement to both the BSO and the Tanglewood Music Center, the school for advanced studies. He enlivened BSO programming and inspired students with his rehearsals, performances, master classes and all-round presence. The school has remained a lively place since he left, but it lacks a commanding artistic figure at the top.

In retrospect, BSO programming over the last two years made it clear that Nelsons, along with Stéphane Dèneve and Daniele Gatti, was one of three candidates receiving closest scrutiny. Each was brought back for multiple performances (which, at the same time, became auditions).

Nelsons, who has conducted at the Met, doesn't come with the instantly recognizable profile that Gustavo Dudamel brought to the Los Angeles Philharmonic when it went shopping for a director a few years ago. Nelsons is probably even less known in the United States than Yannick Nezet-Séguin, whom the Philadelphia Orchestra recently picked.

Name recognition can wait, and we can probably expect a publicity blitz to spread the BSO buzz beyond musical circles. Meanwhile, senior guest conductors such as Rafael Frühbeck de Burgos and Christoph von Dohnányi will provide continuity and depth.

Nelsons inherits an orchestra in fine technical shape -- paradoxically, not through a director's vision, but by having to adapt to a myriad of musical approaches from a steady round of guest conductors. This summer, his flair for drama should be on full display in the hot-blooded Verdi Requiem, his only appearance.

The world changes. The BSO changes with it.