What I most often find lacking in dubbing (in general) is that the actors just don't seem into it..They really don't show emotion...they lack intensity.
For instance, Van's scream in Escaflowne the move was..so intense..so raw..you think they were skinning the actor alive..it actually sent shivers down my spine! The english dubbing there is good, but it lacks the intensity.
That said I havn't heard the original Iroh voice in english (heard the season 3), but I guess it was better than the current one (which imho sucks). Aank, Katara and Sokka voices are ok, but IMHO german voices are better..Zukos voice is the worst I've heard so far in the series.