Examples might not help you much. You admit being a casual reader, and there's nothing wrong with that, but if you're not a re reader who paid a lot of attention to each of the secondary character's voices, I doubt you noticed what was off or missing or that you would care much. The Brandon books are massively better received by the casual fans like you and that's ok. The more invested fans who paid a lot of attention to details (and Jordan was a rewarding writer for those, as he was extremely consistent with the details and character voices) are understandably far less satisfied with the result.

A lot of characters were simply off. Their body language was different, the tone of their lines was different, the things they focused on during their POVs was off. They felt like re-cast actors going through RJ-planned scenes. Good examples of those would be Cadsuane and Tuon, or Romanda and Lelaine. To a lesser measure, there's Elayne.

RJ was a devil for details. Brandon threw in many trademark RJ things, but often they were used just in the wrong places, and that's noticeable for those who paid attention.

Some characters were interestingly written, but they became more Brandon's characters than RJ's. Pevara is one of those. She was interesting, but it wasn't Pevara.

For others, it's more subtle. They're for the most part not really "off", but some layers of their personalities or that made the flavor of their POVs were gone. They became thinner, more one-sided or reduced to a few core elements. Egwene is one of those, and I bet it owed much to the fact RJ left for her a lot of unpolished first draft dialogue and Brandon used it more or less "as is" without adding the trademark Egwene stuff that RJ would have added in further drafts once the core elements of the scene were laid out.

For other characters, it's more like breaking continuity in their character development. For some it's as if Brandon hit rewind, and made them go through late series phases again, in new scenes. It's like Brandon did this to get into the characters, which is fine, but normally those "writing exercises" should have gone to the cutting room floor afterward. Eg. of this: Perrin in TGS and early TOM.

With some, Brandon struggled more to find the voice, and didn't have that much to go by (they didn't have many POVs before) and they came up a bit suffering from the KJA/"Dune" syndrome (ie: Brandon re-hashed old stuff or clichés from their culture to write them, making them feel a bit like caricatures). Aviendha is one of those.

For others it's not so much their voices were completely off on the whole but that Brandon made one big continuity error then ran himself into walls with them. Tam and Uno are two examples. Both have mini-arcs in AMOL that are based on such continuity errors. To those who spotted the errors, their arcs made no sense.

Then there were those who were completely off. E.g. Graendal or Mat.

Ituralde is another who started fine in TGS but gradually veered away from the established character to finally become just cardboard. It was no longer so credible as a Great Captain.

Mat was the most off, and it was the most harmful because he's so central.

Overall, Brandon also dealt with the secondary cast in a very un-RJ way. In his hands they were less characters but cardboard puppets bearing their names, when he remembered to include them at all. For the most part Brandon didn't bother, instead he created picket fences with names to fill those minor parts. It's noticeable in both TGS and TOM that in the beginning of the books he paid more care to include "real players" and the more he progressed, the more he started either re-using a single player over and over (eg: Naeff in TGS) or start to introduce tons of picket fences (all those new Asha'man, AS, warders, Two Rivers people, Aiel...). I guess for more casual readers it didn't matter as they couldn't tell them apart from existing minor players, but when you can it did matter.

It's perfectly understandable he did that in first draft, not to break his writing momentum just because he had to have two maidens stand guard in a scene he'd need to name or add a line or two of dialogue for later. I'm sure RJ did much the same. Leaving it at that in further drafts though, when he had two assistants who could easily find him and provide him with the right players and a few backgrounds elements to use so he could make these "utilities" into established minor players in further drafts gave the three books a very rough and unpolished feel.

Brandon did a fair job, more than fair, for the casual readers, and it's quite OK they're happy with it. It's not as nearly satisfying for the more invested readers. Jordan had caught us in his webs, and paid us back with great consistency. We could trust that our attention was rewarded. Brandon sort of let us down big time in this respect.

Much of that is less Brandon's fault than the circumstances he wrote in. That doesn't change the fact to the more invested readers the books feel terribly unpolished and, in various small ways that in the end piled up, unsatisfactory.

Overall it's not that it's all bad, it's that too little time or efforts were given to take what Brandon wrote and bring it to the next level and polish it up.