I can think of the following (I am assuming you are talking about time estimation)
- The task itself: How long does a similar task usually takes? 1 day? 2 days? A month? How complex is the task? Do you expect to see any uncertainties or does it have any associated risks? What about its precedessors? How many predecessors does it have? Is it on the critical path?
- The team member who's doing the task: Is he fast or slow? Does he understand the task, or is it easy to explain the task to that team member? Is the quality of work produced by that team member good, or does the work need to be almost completely redone after him?
- Is there more than one person doing this task? If yes, then there are more communication channels, which means that there is a bigger communication overhead, and you have to account for this.