When DJing from a laptop, the software needs to have these features which are above and beyond the traditional music player.

• Preview on Headphones, enqueue to Public Address system playlist - dual sound cards
• Preview immediately on headphones, enqueue to Public Address system.
• Folksonomy/Tagging - What dance can be done to each song, favorites, song that need to be re-ripped
• Beats Per Minute Sorting
• Volume Normalization. Don't make your dancers jump when they hear a newer song. See Loudness War.

MpDJ does not have:

• Real-Time tempo adjustment
• Real-Time sound effects
• Real-Time mixing of multiple tracks

To Add:

• Advanced playlist support.
• Advanced search functionality (and, or, tag:).

For this functionality, I'd suggest looking at Mixxx, though it didn't seem fully stable to me as of Nov 2008.

Screenshot