Music DJ Studio not perfect...
It is OK for messing around with, the software i use for making music probably has less samples that come with the software than this, an upload tool would be immensely helpful, but i noticed you tried that.
I can't really write much more, but maybe dressing it up a bit (graphics and all that)
could really help, a tool witch when the pointer is over something, a box comes up that says what it is. (if you don't understand, hover your mouse over the address bar or the voting icons)
On a baseline, this only needs to be dressed up, access to even more samples, a range of effects, (compressors, pass filters, limiters etc) and the small but make all the difference features. (like the hover tool for instance)
I will vote 4, this has great potential.