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To add to what Jody has already posted, L3 switches often lack features commonly found on routers. Often such features are WAN related. Jody mentioned one common WAN feature, NAT, but they might also lack certain hardware resources, such as sufficient capacity to take one or more full Internet route tables.
Even when running across MetroE clouds, often LAN L3 switches are still a bit "weak", for such usage, which is why Cisco has a series of MetroE switches.
The forgoing doesn't mean you cannot use a LAN L3 switch, just you need to be careful matching what the switch can do with your WAN feature needs.
