Hassan wasn't great in the ring, but he was gold on the mic. He was barley on the main roster for a year, so you can't really rate him in the ring. He was just starting to get upper mid card momentum before leaving the company. Another year or so and he could have been really special. I've never really heard anyone criticize Hassan, but there is always someone on the site.
The question there is whether is was really the guy playing Hassan as much as creative being in love with the character and coming up with a lot of material at the time. I wouldn't call him gold on the mic. He wasn't as bad as Lex Luger was at murdering promos. But he had some fairly stiff live in ring promos. I thought the one he did at Wrestlemania with Eugene and Hogan was pretty lame.
As far as whether it's just some rare guy on the internet hating on Hassan, Deezy is hardly alone. Lance Storm answered questions from fans who thought Hassan was great and has never been complimentary about the guy. Most of his answers usually referred to him as being a poor performer, bad worker, with a bad attitude. The guy trained him in OVW. I think there were interviews out there where he referred to the guy as having a bad attitude during training drills back then.
He might have had the potential to be a decent performer, but it sounds like he was given a high profile gimmick well before he was ready to handle any of it.
Some links to some of the stuff I could find him saying when assessing the character and performer. Not in great detail, mind you:
StormWrestling.com - Commentary - September 18, 2006
StormWrestling.com - Commentary - September 18, 2006