Kalen,
If you're referring to Dave, his argument abilities are very good, certainly his internal reasoning. Perhaps his most-common failing is straw man arguments, but he is no more guilty of that than others, and besides he's a lawyer, which means he does it purposely. They can't help that sort of behavior.
He also throws out circular reasoning on occasion, not necessarily out of a weakness on his part. It's purposeful smokescreening.
But if he wasn't so good at internal argument, his threads wouldn't go on and on and on.
The weaknesses of his arguments tend to come from his premises, which tend to be empirically unprovable. He is very good at reframing argument to shift the burden of proof off him and onto his foe. Or he just ignores anything he can't prove / disprove. He waits those out, and usually those get buried pretty quickly in the torrent of postings.
Bottom line: I'm with Sully on this one...
Mark