Reset the SMC, as per > Resetting the System Management Controller
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
Then if that doesn't help, see > Why your Mac might not sleep or stay in sleep mode
I always use Apple dropdown Sleep instead auto sleep and I too have noticed that my Mac Mini takes way longer to sleep than before Mavericks.
When I checked my Console Log, I get the following messages:
11/9/13 10:52:31.000 PM kernel: AirPort_Brcm43xx::powerChange: System Sleep11/9/13 10:52:31.000 PM kernel: AppleBCM5701Ethernet [en0]: Link down (womp disabled, proxy idle)11/10/13 7:12:37.000 AM kernel: Wake reason: OHC2
There were some 200 new changes in Mavericks and I haven't been able to put my finger on which one is delaying sleep for me. So far I've tried disabling all bonjour and location services, changing my network settings and nothing has helped.