The MLS season commences Tuesday, a week earlier than usual, as the Seattle Sounders, the league’s runaway attendance leaders with an average of more than 36,000, host Landon Donovan, David Beckham and the Los Angeles Galaxy at Qwest Field.

D.C. United, coming off the worst season in its 15-year existence, will open Saturday night against the Columbus Crew at RFK Stadium, the first time the club has begun the season at home in five years.

The New Teams

The arrival of the Portland Timbers and Vancouver Whitecaps increased the number of MLS clubs to 18 and, in the process, created a Pacific Northwest soccer cauldron. Passion for the sport runs deep in the region, as demonstrated in a high-spirited preseason tournament in Seattle. Both expansion teams competed in a second-tier circuit for years and, to meet MLS criteria, upgraded operations and rosters. Montreal will join the league in 2012 and a second club in the New York area is in MLS’s long-term planning.

Colorado’s Quincy Amarikwa and Seattle’s Erik Friberg fight for the ball during a MLS exhibition match on March 9. (Ted S. Warren/AP)

The New Stadiums

While D.C. United continues to toil at 50-year-old RFK Stadium, the rest of the league moves ahead on the facility front. Sporting Kansas City, formerly known as the Kansas City Wizards, will play its first 10 matches on the road before settling into 18,467-seat, $200 million Livestrong Sporting Park, named for Lance Armstrong’s cancer research foundation. A portion of ticket revenue will be channeled to the effort. The Timbers will play at renovated Jeld-Wen Field while the Whitecaps will spend most of the year at a temporary facility, 20,000-seat Empire Field, before moving into renovated BC Place.

The New Conferences

With two West Coast clubs entering the league, the Houston Dynamo has moved to the Eastern Conference. Although the shift separates Texas rivals Dallas and Houston, it doesn’t affect the schedule as all 18 teams will play one another home and away for a total of 34 matches, an increase of four from last season.

The New Playoff Format

MLS added two teams to the league, and then boosted the number of playoff participants by two to 10, meaning 56 percent of the field (and perhaps a club or two with a losing record) will advance to a drawn-out postseason: two single-game wild-card matches, home-and-away conference semifinals, single-game conference finals and MLS Cup on Nov. 20 at a neutral site.

The New Coaches

Ben Olsen, 33, was promoted from United’s interim head coach to the full-time gig. Chivas USA hired Real Salt Lake assistant Robin Fraser, a former MLS star defender. Aron Winter, an ex-Dutch national team midfielder, becomes the sixth leader in five years in Toronto. Iceland’s Teitur Thordarson, with his 12th team in 24 years, is in his fourth season with the Vancouver organization, while Scotland’s John Spencer moves to Portland after serving as a Houston assistant.

The New Players

Rooney to MLS! That’s John Rooney, younger brother of Manchester United superstar Wayne, who was drafted by New York. Charlie Davies, the U.S. forward severely injured in a car wreck in 2009, will try to resurrect his career with D.C., which took him on loan from French club Sochaux. Jay DeMerit, 2010 U.S. World Cup starter, brings his rugged defending and colorful tweeting (@d6merit) to Vancouver. Kenny Cooper, a big scorer for Dallas from 2006 to 2009, joins Portland after time in Germany and England. Mexican national team forward Omar Bravo landed in Kansas City. Carlos Ruiz, the 2002 league MVP, signed with Philadelphia after stops in Mexico, Paraguay and Greece. Colombian goalkeeper Faryd Mondragon is also in Philly, and former Lazio and Manchester City midfielder Ousmane Dabo landed with New England.

The New Conflicts

Last year, MLS shut down for two weeks during the World Cup. This year, there are no hiatuses but scheduling issues nonetheless. Some MLS players will receive call-ups to the U.S. national team for the CONCACAF Gold Cup (June 5-25) and the start of 2014 World Cup qualifying late in the season. Canadian and Central American players are also affected by the Gold Cup, the biennial regional championship. Younger talent, such as United defender Perry Kitchen, will miss time while with the U.S. under-20 squad. Some Europeans will have to return home for continental championship qualifiers.

The New Favorites

Real Salt Lake won the league championship in 2009 and is poised to return to the top with a veteran foundation led by midfielders Javier Morales and Kyle Beckerman, 2010 MLS defender of the year Jamison Olave, goalkeeper Nick Rimando and forward Alvaro Saborio. RSL is also carrying the MLS flag in international competition, appearing in the semifinals of the CONCACAF Champions League.