The problem with Arthas appearing too many times was that most of his apperances made him look like a joke. The first time you see him in Light's Hope? He gets owned by Tirion and looses his largest contigent of new Death Knights. When you see him at the Wrath gate, he gets owned by a gas cloud ('STOMACH ACHES?!' to quote the guy from Vengeance Landing). Zul'drak, not only does he kill Drakuru personally for being a failure but he lets you live (yes, we now know he was letting you live so you could be at your strongest he faced and converted you, but even knowing that it seemed like a massive cop-out). I could list more, but it would become convoluted.

There were a couple of times when the Lich King was portrayed well, like the Howling Fjord quest for Alliance players, the Mattias Lehner quest (even though he gets owned, it happens in suitable specacular fashion and does a lot to show just what makes Arthas the monster he is), and Halls of Reflection, where you don't even fight or weaken him, you just run away like sissies.

The problem is after so many quests with him 'running away' shaking his fist at you, he comes off as more of a Saturday morning cartoon villian than a legitimate threat or a well-written character. They needed to have him with more moments that actually made you scared of him, or dare I say where he all but kils you in a stunning victory.