Bad production/hooks in Hip-Hop music.
Seriously, that can ruin the tracks of the best MCs. One of my favourite artists has fallen prey to this far too often - Nas. I think one of the reasons that he never made an album close to being as good as Illmatic was that Illmatic had producers like Q-Tip, Large Pro, DJ Premiere, and Pete Rock. On top of this the choruses of the songs were made up of scratched samples, like in Memory Lane and NY State of Mind, or, when there were hooks - The World is Yours - they were good! They weren't obnoxious, with some psuedo-melody that contrasted with the beat as a whole.
I think that this - combined with his unfortunate yet inevitable commercialisation - was the main reason that he fell off so badly, well, until Stillmatic - which was excellent. Apart from that, there were only 3 or 4 tracks on his second, third, and fourth album that I'd listen to. The Message, for example, probably the best song on 'It Was Written,' had samples from 'NY State of Mind' in the chorus as opposed to the boring singing of Lauryn Hill.
I'm using Nas as an example but this really applies to most Hip-Hop - that I listen to, none of that Lil Wayne/50 Cent shit or whatever.
I apologise for the tangent though, especially on a site like this where most people don't even like Hip-Hop/Rap.