A 3/4 lifelink for 4 mana is surprisingly
well-positioned in most formats. 4 toughness
gets it out of burn range, black and Vampire
makes it less susceptible to some of the common
kill spells, and it makes your army bigger just
by being on board. Turning all of your
opponent's creatures that die into your own
personal Zombie army can, if coupled with
something like a mass burn spell or selective
board wipe, put you in a unbeatable position
very quickly. And just him being a 3/4 with
lifelink is enough to turn a rough board state
more in your favor.
He can even turn your token Zombie army or your
other forces into more weapons for himself; a
single sacrifice makes Kalitas into a 5/6 with
lifelink, and that's a very tough creature to
kill or race. For his mana cost, Kalitas is
easily one of the best creatures printed this
year.
There's so much to like about this card. With
all the decks in casual and/or Commander
settings that are based around sacrificing
creatures and/or recurring them for benefit,
there's good reason to use effects that slow
down or stop that, and Kalitas is much less
fragile than a lot of creatures who carry his
anti-graveyard effect. That's before you even
factor in how well his activated ability
goes with the themes of Innistrad's vampire
tribe, and effects like proliferate. I'm not
sure you'd pick him over Olivia Voldaren as the
general of a vampire-themed deck in Commander,
but they would certainly work well together in
such a setting.