His main problem is the simple rhyme schemes. His lines are basically long sentences that rhyme at the end. He also frequently makes long pauses (a criminal offense in hiphop, IMO).
Allow me the use of a simple example to show you the difference between him and an amazing MC (rhymes are marked as -):
> Macklemore:
I call that getting tricked by a business, that shirt's -hella dough-
And having the same one as six other people in this club is a -hella don't-
Peep game, come take a look through my -telescope-
Trying to get girls from a brand? Then you -hella won't-
> Nas:
It ain't hard to -tell-, I -excel- then -prevail-
The mic is contacted, I attract -clientele-
My mic check is -life or death-, breathing a -sniper's breath-
I exhale the yellow smoke of buddha through -righteous steps-
Deep like the -Shining-, sparkle like a -diamond-
Sneak a Uzi on the -island- in my army jacket -lining-
Hit the Earth like a comet –invasion-!
Nas is like the Afrocentric -Asian-, half-man, half--amazing-
Cause in my physical I can -express- through -song-
Delete -stress- like -Motrin-, then -extend- -strong-
I drink Moet with Medusa, give her shotguns in -hell-
From the -spliff- that I -lift- and -inhale-, it ain't hard to -tell-
---
Also note the abundance of alliterations in Nas' quote.
EDIT: fixed formatting
EDIT2: couldn't resist doing the whole verse for Nas. Illmatic is so good...