Segment by timestamp and/or VisitorID

Good request! We'll monitor this for votes and comments. 

As a note, there is an "Hour" dimension in segmentation, but it currently refers to a specific hour on a specific day (e.g. 2015 Feb 27 8:00). I think you're saying that you want to be able to segment by (for example) all 8:00 hours across all days in the data set. If so, that makes perfect sense to me. 

We achieve this by using the get TimeParting plugin which captures in 30 min buckets, which is then re-classified into hourly buckets (and any other definition such as your definition of morning, afternoon etc)

You can then use to create the segment .. ideally this should be out-of-the-box without needing to use third party plugin (and then having to maintain)

@Mubs: Good workaround! You can also copy the visitor ID into a variable, although I will be the first to admit that it's less than ideal to burn an eVar on something like that (and it also assumes that you have fewer than 1 million visitor IDs per month).