I'm new here at RTM, and I am loving Smart Lists to set up my GTD system. I have been able to create lists to filter out all my tasks that:
I cannot start yet
I don't want to start yet (do someday)
Items I have in RTM for reference only.

The items with future start dates will automatically show up in my list on the start date I have assigned.

Most of the time I want to look at what I can do now, based on whether I am at work, in front of the computer, or doing housework, for example. In GTD, these are called contexts. I have tags for each of my contexts (@computer, @phone, @home, @work, @errands). I created Smart lists to filter for these contexts too!

I have my Next Actions list - all my tasks that can be worked on, any context. This filters out any tasks that have a future start date or are tagged someday or reference. I don't create a start date for every task, but there are certain tasks that I can't do now, and I want to remember to do them when the time is right.

Here's my search query for all my Next actions:
startBefore:Today OR start:Today OR start:never NOT tag:someday NOT tag:reference

In addition to filtering out any tasks that have future start dates, someday or reference tags, I filter by context, so I can really focus on what needs to be done depending on time and place.

Here's my search query for my @computer context:
startBefore:Today OR start:Today OR start:never NOT tag:someday NOT tag:reference AND tag:@computer

I haven't used locations much yet, but I am excited to see how that can fit in too!