Sure, it'll be nice to have a new class -- we've been stuck with the same nine since 2004. However, there are some downsides to the Death Knight as well. Some see it as a threat to their raid spots (mostly rogues and warriors), some don't think it fits in well with the lore, some are afraid it'll be overpowered. However, I have a different concern: how am I going to fit one in my character list?
Some players with less severe altitis may not know it, but there are limits on how many characters you can have: ten per server, 50 per account. Ten is a good number for the per-server limit, at least right now; you get one of each class, plus one more for a bank/AH alt. But what are we supposed to do if we want to make a Death Knight? Delete one of our beloved alts? Pay to transfer a character? Start the DK on a different server (if that's even possible)? CM Nethaera tells us there are no plans to add more character slots, although she notes that people are unhappy with it and that she'll relay that to the devs. Are you going to run up against the character limit in WotLK?