As I've before, my only criticism of Leslie (albeit minor) is a lack of vocal range - many of her characters sound like a variation of Abby. But then again, Frank was kind of the same way in the past, many of his one-shot and incident characters sounded like variations of Bert.
As for Two-Headed Monster, since they've always basically spoke in monster gibberish, I think it's a little easier to overlook changes in their performers, even though there are noticeable differences: Jerry and Richard gave them deeper, gruffer, and somewhat more subdued voices, whereas Joey and David sounds like Baby Bear and Papa Bear with sore throats after having eaten porridge laced with speed.
As for Two-Headed Monster, since they've always basically spoke in monster gibberish, I think it's a little easier to overlook changes in their performers, even though there are noticeable differences: Jerry and Richard gave them deeper, gruffer, and somewhat more subdued voices, whereas Joey and David sounds like Baby Bear and Papa Bear with sore throats after having eaten porridge laced with speed.