There are many factors to consider. The first would be whether or not your old company will allow you to remain in their plan. If you balance is below a certain threshold, then they may roll your balance out or force a distribution (including sending a portion of your distribution to the IRS if the amount is over $200).
Another factor might be when you are eligible to participate in your new company's plan, or if they even have a plan. How much does ease of investing and planning mean to you? Do you have help working on allocation and planning issues for assets in the old plan or the new plan or perhaps with an independent advisor. If I had a good plan, low costs, good diversification, and the plan was not too difficult to maintain at the old employer, I might be tempted to leave it there, but if there are better options in the new plan, then I'd begin to consider that as a better option.
Great points Billy! I have found that leaving money in an old employers plan after you have left may cause you to incur a monthly management fee so be on the look out for that as well.
Rolling your account out of your old plan and into an IRA to be managed by an Advisor might be a good option if you have little experience with investing and are looking for personalized guidance to retirement. As Billy said though, if the new plan has good diversification, low cost and you believe you can manage the investments on your own, it may make sense to roll it over to the new plan.