Unfortunately I don't know what's wrong. Maybe if you can say exactly what you did someone will be able to spot the flaw.
Meanwhile, I guess the next "silly question" would be did you wipe before installation? I did install the non-A2SD version of LeeDroid once without wiping, and got away with it, but the A2SD is further from stock and so wiping is more likely to be necessary. So in the absence of any other ideas, my next thought would be to go into recovery (hold the volume down button while the phone boots, then follow instructions on the screen), select wipe/factory reset, then select "install zip from sdcard" and try to flash the ROM again. After that you should be able to restore your apps and data from titanium/mybackup.
A nandroid is different from titanium and mybackup - it backs up your ROM, apps and data as a single package, allowing you to restore the phone to exactly the state it was in when the backup was taken. Titanium and mybackup only back up apps and data. All have their places, but nandroid is always a good idea before flashing because if something goes wrong you can use it to restore from recovery and get back your phone as it was before the flash. The others are only useful once you can boot the phone again.
