The speed is better, vastly better program organization and execution/priority control, the new Memory view (like AB, Omron, Idec, and everyone else has, finally, but better!), more and better types of memory, user definable memory (!), typed memory (still castable when you need to), protected memory that can only be accessed with secure comms protocols, the awesome Trend View, the (way past timely) death of BCD, I could go on and on. It has BOTH tag/heap and conventional table memory.
I actually hate when I have to work on a DL-classic because there's so many improvements in DM. I haven't installed a DL205 since DM came out, nor do I expect to.