Ammon News of Jordan has a couple of articles about Jordan's uneasiness with the Syrian situation.
The idea of Islamists taking over Syria is a major concern. Right now Syria is Jordan's major trading partner and many Jordanians in the north are related to Syrians. An Islamist takeover would strengthen the Muslim Brotherhood in Jordan, and the MB would try to unite with Jordan being between their centers of power in Syria and Egypt. It also has the potential of wrecking Jordan's already shaky economy. Syria is Jordan's gateway to Lebanon, Turkey and Eastern European markets. This is one reason why Jordan does not support economic sanctions against Syria.
On the other hand, the massacres of civilians in Syria bother most Jordanians and the government as well. King Abdullah was among the first to call for Assad to step down.
Their close ties make it inconceivable that Jordan would withdraw its ambassador, though.
Another complicating factor is that there are thousands, and perhaps tens of thousands, of Syrian refugees in Jordan now.
King Abdullah reportedly does not believe that Syria is going to fall in the short term, but another major concern is that when it does, it will become another sectarian-torn mess like Libya and Iraq.
Any way you look at it, Jordan will be in a worse situation in a couple of years than it is now - and it is not in great shape now by any means.