Well, just to break it down to bullet points:
* Acting: Terrific, top notch. They really got into their characters, making them both similar and disimilar to their normal counterparts.
* Writing: Excellent, excellent, excellent. Sussman's said in the past that this was a real labor of love for him, and it shows in how it turned out.
* Visual FX: Egads, the one real weakness of the whole show. The Imperial guy on the moon, the Tholian, the Imperial paintjob on the ship, all looked terrible. The Defiant was adequate, if innacurate, but nothing spectacular or special. Score one for the "models are better than CGI" crowd. About the only thing that looked nice was the Tholian web, but given how simplistic it was in TOS, that'd be the hardest thing to get wrong.
* Set design: Where the VFX people failed, these guys succeeded and thensome, bringing their best to the table by not only redressing NX-01 for the Mirror Universe, but also recreating a Consitution-class ship interior from scratch. These guys deserve an Emmy for set design.