With Saints Row 2 around the corner, I'm trying to finally finish my 100% run on GTA IV.  I'm just about done with the biggest single chunk, tracking down all 200 pigeons hidden around the city and exterminating them.  While it isn't quite as fun as the main story missions, it is still quite enjoyable -- really it provides an excuse to poke around the various nooks and crannies of Liberty City.

They really made a lot more city than they have game for.  I am constantly coming across some cool plaza, hidden park, or interesting architecture that is more or less completely bypassed in the main game.  While the DLC coming later this year is likely to add some new areas (I'm guessing another small island or two), honestly they could add a lot more gameplay within the core areas they already have and it would not feel out of place.

Ultimately, GTA IV is unlikely to displace San Andreas as my favorite in the series.  But like San Andreas, it is increasingly becoming a place I just like to visit and drive around in.