Fallout 3. I originally picked it up wen it first dropped on rental, and there was virtually no information out there. I was looking at Gamefaqs & wikis and all that, and seeing that it appeared there was nothing out there. I got bored with it (since I'm one of those completionist guys), and eventually came back to it six months later r so, and boy had things changed.
Also, World of Warcraft. I'm pretty sure everyone who's gone through on any number of toons can tell you there's parts of leveling that just slog. For me, it was my Feral Druid, between 10-50. 20 made things a little easier with Catform, but until I got Mangle at 50 it just wasn't cutting it. Haven't looked back since. Also, on my Warlock (Affliction) there were a few slog points, mostly revolving around the new minions you could get at each level and how much they sucked compared to the Voidwalker. Things perked back up around 30 or so, but the grind from 58-60 as capped off by getting the super-fun Haunt, which has made leveling the guy a breeze.