I'd originally planned on playing through the GDI campaign of C&C 3 on the PC, then the Nod campaign on the 360. Unfortunately for my plans, unlike the PC version, on the 360 you have to unlock the Nod campaign by first playing through the GDI campaign. Ah well. I figured this was as good a chance as any to get the Achievements for playing the campaigns through on the hardest difficulty and completing every bonus objective.
I'm afraid that in the end, the difficulty of using the 360 controller will prevent me from reaching this goal, but a few missions in it seems manageable. Really forcing myself to do the optional objectives does put a different slant on the game. Normally I'm not big on replay through difficulty but since I'm a victim of circumstance here, I might as well make the best of it.
The games with the best replay value are ones where you can really explore different facets of the environment and game mechanics. Achievements can also provide an interesting structure for alternate goals. Dead Rising is a great example of this -- playing through the game to get all the "PP" bonus photo stickers is pretty different than trying for a perfect survivor rescue. Alternate endings by themselves I don't find overly compelling, but since DR has enough structure to compel replay in the first place, the endings are a nice bonus.
Really compelling DLC, like the ones announced for Crackdown today, also help. They don't encourage replay, per se, but they are going to get me playing the game again..