My HTC Desire HD has been stolen. What data can still be accessed by the thief? I have contacted the network and had the phone barred, I had applied a screen lock.  I am concerned however if the thief can still access all my numbers, texts, saved passwords and most importantly the pictures/videos of my daughter which were stored to the handset?  It terrifies me to think that some horror could access these for all kinds of use.
If the data was not encrypted: everything.

The data on the removable SD-card can be easily accessed, just like a lost USB-Memory-Stick. The internal flash storage can also be read out via some, admittedly sophisticated but practical possible, approaches.
AFAIK no Data that you sync with the Google cloud is encrypted.

It's always wise to install some remote wipe tool, like pyko said. Even if they are not foolproof and can not provide a "secure delete" functionality, they provide some security and raise the bar to read out your data.

Hint: The AN.SX security tag provides some further related information for such cases.