Based on the rappers I've worked with and the not so great beats they've brought in compared to the pro's:
1.) The beats are written with some clue to what music theory is.
2.) The beats aren't a stereo submix, allowing you to actually mix, compress and eq the individual tracks come mix time.
3.) Pro's don't try to record 2-3 songs in one hour and have no mixing time at all.
4.) Most pro's aren't writing their lyrics 5 minutes after getting to the studio.
5.) Not having most of your tracks clipping is always nice.
6.) Not using a bunch of weak 8 bit samples found online is another plus.
7.) Pro's will actually use real Minimoogs, 808's and so forth.
8.) In general the pros are probably spending 20-30 hours more time than your local MC's to get it right.
9.) Pro's hire the best songwriters and sound designers to work on their stuff, instead of buying 50 dollar beats from PhatDoggBlunt.com productions.