I think it is important to say, that i don't think Warcraft characters get automatic better by having been in Warcraft 3. It is more about the focus given to the character, that they often are given a voice and a personality. Even Vol'jin as a character, while he got little screen time, got his own voice and lines which showed characters.
And the lack of voice and lines is pretty much what makes many new Warcraft charcters lacking. Blizzard have a problem with not being able to focus on individual characters in the extent that is needed. Sure, some characters get their 15 min of fame, but it rarely shows much less then their attitude to a specific problem and not much of their character. This has lead to Blizzard creating alot of characters who seem pretty shallow to the player, which makes it difficult for the player to make an emotional connection to these new characters.
Blizzard could really help to fix this by making more character focused questlines. Like if we got a new troll leader, Blizzard would have to atleast dedicated 1 or 2 story-lines where he or she is either the main character or a strong supportiv character. NPC-interaction could also help alot, especially when it comes to clarifying strenght. Prince Farondis quest line is a good example of how to show power and also character by having the player fight beside the NPC.