One the one hand, he's frequently described as the prototypical "average" big league first baseman, with his 162-game average of 26 home runs, 93 RBIs, and .271 average. On the other hand, his career similarity score of 940 on baseball-reference.com is the same as Adrian Gonzalez. That's a deceptive number, however, in that Gonzalez is a three-time all-star and LaRoche is still waiting for that first All-Star selection.
Defensively, LaRoche has a career fielding average of .995, just a point behind Mark Teixiera, yet I doubt anyone would rank them as near-equal as defenders. LaRoche is no slouch, but he's not a Gold Glover either.
What he is, however, is a real first baseman, not a project. He'll save some runs with defense, and certainly he'll keep the throwing error totals for the left side infielders down considerably, I expect. Offensively, he's a complimentary player, who will drive in runs, but will also strike out 150 times and walk 50-60 times. He puts the ball in play, without hitting into a lot of double plays.
LaRoche has bounced around a bit, and in that sense, is a journeyman. He gives the Nationals something they need right now, and likely understands he won't retire in their uniform, barring injury. He also knows he wasn't their first choice, and at 31, needs to repeat or better his numbers from 2010 to justify his signing.
Sometimes, "average" is acceptable. The Nationals hope Adam LaRoche turns out better than that, but 25 bombs and 100 RBIs will help stabilize the middle of the lineup, and saving a dozen or more throws in the dirt helps polish the defense.
They'll take that in a heartbeat.