+1 on the wheels, but another thing to consider are the tires also. Going from a wire bead to a floding kevlar beaded tire can take a bit off the bike. Also, a belted tire that has kevlar in it to stop flats or reduce them is nice insurance to have. The Bontrager Hardcase tires are very good for this while still maintain some performance.
A second thing to really look at are your wheel hubs. Have they been serviced lately? Are they properly adjusted to get the most out of them? So many bikes come with cones slightly out from where they need to be. Then when things are tightened by the skewer, it binds up the bearings just slightly and causes some added drag. If someone has a bike nearly close to yours, do a coast-down test side by side and see which one coasts better and then try and figure out why? Does one tire have MORE air pressure that the other? A LOT of little things can make a difference.