If offers different features. For example of one, see this whitepaper which explains a QoS feature not availble, I believe, on the non-Metro 3750s: http://www.cisco.com/en/US/products/hw/switches/ps5532/products_white_paper09186a00801eb831.shtml

From http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5532/prod_qas09186a00801eb822.html:

Q. What is the difference between the Cisco Catalyst 3750 Metro Series and the Cisco Catalyst 3750 Series?

A. The Cisco Catalyst 3750 Metro Series is built for Metro Ethernet access in a customer location, enabling the delivery of more differentiated Metro Ethernet services. These switches feature bidirectional hierarchical QoS and Traffic Shaping, intelligent 802.1Q tunneling with class-of-service (CoS) mutation, VLAN translation, MPLS, EoMPLS, and Hierarchical Virtual Private LAN Service (H-VPLS) support, and redundant AC or DC power. They are ideal for service providers seeking to deliver profitable business services, such as Layer 2, Layer 3, and MPLS VPNs, in a variety of bandwidths and with different SLAs. With flexible software options, the Cisco Catalyst 3750 Metro Series offers a cost-effective path for meeting current and future service requirements from service providers.

I believe the hardware also provides redundant power supplies within the box.