First of all. Ammo is important. REALLY important. If you're support, hit 3. A LOT.
Secondly, the Support class is a master of laying down suppressing fire, and can keep people from popping out from behind that corner while the rest of your team moves up, or flanks them.
Thirdly, the support class later unlocks C4's (very handy for taking out tanks and other armored vehicles), claymores (useful for keeping your team from being flanked) and a mortar (very useful for defending an mcom or providing support for your squad).
To play support effectively, all you really need to do is deal out ammo around your team-mates. A support that actually hands out ammo will be noticed by others and commended (my proudest achievement so far is the resupply medal :P). Other than that, keep chokepoints bullet-ridden, and take out any armor that comes along (if you have c4).
All in all, while support may seem like it's got less to offer, it's actually a much more versatile class than any other, able to deal with almost any problem that comes along.