In Live, when you trigger a new clip on a track, the old clip stops instantly.
In ProTools/DP/Cubase, when you "trigger" a new clip, you can let the old one fade out gradually, and for X number of seconds, both clips are playing simulataneously. And instead of "triggering" the new one, you can let it fade in for X number of milliseconds before it "starts".
This is useful for when you, say, record a guitar part... and there's a spot where you messed up so you go re-record that one section, but leave the rest of your otherwise-good take. The splice points are smoothed over by the crossfade (just like in Simpler/Sampler-- which uses crossfade to smoothen the looping function). The assembled guitar track only takes up one track-- which is not only visually pleasing, but means you can apply just one EQ, and one reverb, and/or one compressor to make your track sound good.
When you have 10 clips all stopping at the same time, and 10 new ones starting (as you transition from verse to chorus, perhaps) I find that Live hiccups a little. There are workarounds, but I'm used to the crossfade-method. Other ProTools users might feel the same way.
Its a basic function on all other "linear" DAWs I've used (ProTools, Cubase, DP), but isn't implemented in Live 6.0.5. Maybe in Live 7
Even if it isn't, I'm not going back.