In light of the Bay Bridge closure, BART service that runs 24 hours remains a pipedream dancing in the heads of almost every public-transportation commuter. Why can't it be this way year-round? Well, on their Labor Day Weekend Bay Bridge Closure page, BART explains why they cannot offer 24-hour service:

BART does not provide regular 24-hour service because many safety-sensitive, essential and/or California Public Utilities Commission mandated maintenance work can only take place when the trains are not running. The 24-hour service during the Bay Bridge closure will mean that BART will have to play catch-up on essential maintenance work. Unlike some public transit systems with multiple sets of tracks on the same routes, BART doesn't have the duplication that would allow us to run trains on one set while performing maintenance on another.

Now you know.

Speaking of maintenance and last night's 24-hour kickoff, SFist has already received several emails from angry commuters complaining about the time it took to get from the East Bay back to San Francisco in the early AM. One reader, who shall remain anonymous, writes:

"File under: pure nimrodism or ploy to get overtime: On the first day of the planned 5 Bay Bridge closure in order to sync up the new spans, I went to the East Bay for dinner on BART. On my return to the city, the train stops at the entrance to the transbay tube, and its announced that there will be a ten-minute delay because of maintenance on the tube! Maintenance that requires single track use, so only one train at a time can go through instead of one going in each direction. This when the entire Bay Area is basically forced to take BART if they want to get to or leave SF. "