The only way to end that 70s show is to end the 70s, so this was a logical choice for the last episode of this hilarious show. While the timeline has always been confusing (having 5 or 6 christmases in about three and a half years), it never really matters, because all that matters is that the show is as entertaining as ever on the last episode, and this one does just that.
It had some great callbacks to the rest of the show like the stupid helmet, foot in ass, the water tower, Hyde's most told "trip" story of the car that runs on water, Red's love for the packers, dumbass and of course Eric and Donna's relationship, and while they could have done more, such as a final "I said good day" from Fez, as long as there's a last circle with the gang, it was bound to be a good episode, and it was very funny, with Kelso and Eric back in it, and Kelso wanting once again to light something up, as well as wrapping up the main story points of the show, without trying to act like life ends after the episode.
Kitty and Red not moving seemed like a waste of a few episodes, but it was nonetheless a funny plot, and not showing anything after the turn of the decade was a wise choice. Overall, this episode definitely feels like a finale, and while there are a few faults, the show couldn't have ended any better, with hilarious lines from everyone, and the gang back together again, however briefly before Kelso goes back to Chicago with his rhino horn, Donna goes to college and Hyde gets busted, but the mystery at the end works for the ever changing plot of the show, and while the eighth season is possibly the worst of the show, this episode is one of the greats.