Halloweentime has, in the past 3 years, become one of DLR's biggest holiday celebrations! With Space Mountain and Haunted Mansion having special Halloween themes, plus the special Halloween fireworks and all of the decorations...this has also led to an increase in crowd levels, especially on weekends.
And although weekend crowds are much higher than weekday crowds, you will want at least one weekend day at the parks so that you can see the fireworks/Fantasmic that usually don't play during the week in October. By planning and reading tips from these boards, you can weave around crowds pretty well.
Weather is usually pretty good during October (sunny and relatively warm), but with weird So. Cal weather patterns recently, it's hard to tell.
Oh, also be warned that there may be a ride or a couple rides down for refurbishment during October. As it is still kinda "offseason", this is the time when DLR decides to close their rides for maintenance.