Question for the UK

As much as I love hearing David Tennant whisper my name (or, more
often, yell it out in utter frustration), I am not in love with the
Doctor's new companion.  I'm guessing from the hype that landing
Catherine Tate for the Xmas Special was some sort of big deal --
enough to warrant giving Freema the bum's rush out the door to
Torchwood to make room for her in the Tardis as a regular -- but even
as a big fan of imported British television, she's meaningless to me.
Can someone put her into context for me?  What has she done and why is
she a big deal?  (And why shouldn't I keep wishing she get run over by
a passing bus, chariot or spaceship?)

Thanks!

Donna