Well, yeah, it's not difficult. In fact, according the defensive spectrum, it ranks first base as just above designated hitter in terms of difficulty. That's not to say first base is not without its nuances that some players find hard to adapt to. Put Ryan Doumit in that category as well.
According to 1500ESPN's Judd Zulgad, here's what the new Twin said:
"There's more than meets the eye with playing first base," said Doumit, who is scheduled to catch Scott Baker on Wednesday in Port Charlotte when the Twins face Tampa Bay. "A lot of people just assume that you go over there and catch the ball when it's thrown to you. But it's difficult. I've played probably about 30 games over there in my career and it's really not one of my favorite positions. Of my list, I would probably list that at No. 4 of the positions I'm going to be playing. I'd probably rank that one my least favorite. It's tough. It's a position that takes a lot of work." Asked to elaborate on particulars, Doumit said: "Getting adjusted and if it's not something that you've done (a lot) it's kind of tough to get it down. The little intricacies of getting your footwork in position. From taking a throw from third, shortstop, second base. Knowing where you need to be on cutoffs and relays. There's a lot that goes into it."
Doumit's track record at first is just 251 innings long and 208.1 of those were accumulated in 2006. One of the biggest things a high-quality first baseman can do to contribute defensively is scoop or pick throws in the dirt. A good first baseman is quite adept at this. For example, Carlos Pena for the Cubs picked 58 balls out of the dirt for his infielders, saving his team numerous base-runners. In 2008, Justin Morneau led the majors with 47 scoops. While that was over the course of an entire season and over 1,000 innings, Doumit has not been on the same pace, even in the season he worked 200 innings. In 2006, he managed one scoop. Admittedly, this leaves out the data on how many balls he had the opportunity to scoop or pick but given that he had 200 innings, you can assume that it was likely a fairly high amount.
Part of that is footwork and being comfortable at the position. Part of that is a skill.