I think the poster who called LiT a decent ships-passing-in-the-night story go it right.
The concept of it, I grokked immediately and realized where it was going: an "artsy" film trying to portray "really real life, unedited". With the assumption in place that really real life is kind of meandering and, well, boring. It reminds me of Cowboy Bebop believe it or not: part of the atmosphere of Bebop was tied to the characters having long, "normal" moments of semi-bored silence and not much to really say to one another other than the same old "hi", "how's it goin'?" each day.
Bebop however, uses those moments strategically. LiT tries to make an entire story out of nothing but them to reinforce its point. Personally, I think that only partially succeeds. I think that it <i>is</i> boring in the way that a lot of artsy films are boring because they're so "representational" and faux-zen. "Now, let us examine the empty parking lot for 20 minutes. See, as the Phillip Glass score reaches a discordant climax, the ant crawl silently along the yellow stripe."
The emotional punch in LiT is very real though, I think, and part of what saves the movie, such as it is saved. Bill Murray was the ideal choice for leading man, though I do have to agree leading actress was a bad choice for me and entirely uninteresting.