I've been reading a few technical books on various music styles, and one thing that hasn't been mentioned is that you can be "in time" but where you're playing with the beat can dramatically change the feel of what you're playing.
Play with a metronome, and try to hit exactly when the metronome goes off. If you're feeling snazzy, record it and play it back. Now, do it again, but let the metronome lead -- your notes should come in a split second after the metronome. It'll lead to a lazier, slower feel -- without actually changing the tempo at all.
If you're rushing a metronome you're usually hitting before the actual beat. That's fine if you're playing in a punk band or ska, where the music has a lot of energy and speed, but a lot of that energy and speed
feel comes from where guitarists are starting their strum, and the same is true for bass -- when you pull that string, you should be doing it the same way for each song.
But exactly how you approach that song is up to you. For example, if you listen through Miles Davis' "Kind of Blue," you'll note that he has such command over where he wants to put every note that, if you were to put a metronome on the track, he almost sounds late or wrong. Take the metronome off, though, and you hear something different -- that he put each note where he intended to.
So playing a little behind the beat for a lazy passage, and then playing right on top of the beat when it goes doubletime, well, that's what practice is for
For something more modern, you can hear some good examples of playing around with groove on Soul Coughing's "Ruby Vroom" album. "True Dreams of Wichita" has some very "lazy" bass playing -- he never loses time, and he never conflicts with the drummer, but the note usually hits later than "perfect" because that's how most of the song goes. And then there's this part where it gets really funky and he's playing opposite the drummer and he's right there on top of every beat, nailing it.
Bass is mostly a rhythm part, and understanding how that works is really key to making it work. My bad habit is that I don't practice with a metronome all the time, when I really should be. Understanding how to go from playing "perfectly" to actually expressing the music with your playing, though, is what comes after you understand timing.
edit: a perhaps overkill example are the various versions of Satie's "Lent et douloureux" of his Gymnopedies. As written, they're simple and sweet, and played exactly as written, kind of boring. Most versions will seriously "drag" behind the tempo, though, almost to the point of being late, and that's where the feeling comes in -- understanding where the notes SHOULD to go, even though they're written on the page ;D