One funny thing about the game: DIII is a mix of newbies and old coaches who camp out there to prey upon the newbies. I'm not saying that it's "easier" to win at the DIII level, but you tend not to have a large group of coaches stay there too long, as people desire to move up. Which means that DIII coaches who know what they're doing can build a dynasty there a bit easier.
DII is a lot of fun--coaches are more "experienced" and recruiting is more fun at DII level than just about any other level.
DIAA--I'm not going to lie--is a real challenge. In large part because there is more significant overlap between recruits with top end DIAA guys and lower end DI guys. That means that even if you do a great job recruiting, if you are chasing the top DIAA prospects you can almost assuredly count on marauding DI coaches who've struck out on their prospects to come calling on signing day, which wreaks havoc at the last minute. It can be very frustrating to have a class lined up, only to have your top guys plundered at the end--and tough to scramble to replace those guys with not much $$$ at the end.
DI is an absolute grinder. Not all, but the vast majority of coaches know what they're doing, and the top programs usually have very solid coaches. Many of the teams are stacked on top of each other, which makes it tough to compete on the recruiting trail. You also have "elite" teams that can bully the others, because they get a bonus "boost" in recruiting due to the prestige of those programs. If you are at a smaller DI school [say, a school in the MAC], it can be tough because you're right in the middle of B1G country and you just can't compete for the same players. But if you make it to the pinnacle, DI can be an exciting challenge.