The finale for me was quite an underwhelming and disappointing affair. Very average and lacking.
The mythology was all but ignored. The action was poorly executed which is astonishing when you look back at all the great action sequences the show had over its run. The fight between Smokey and Jack couldn't have been any more anti-climatic.
We waited 6 years for the mystery of the island to be revealed as a plug and a lame light show. That's the best they could come up with! Jack's brilliant decision-making involved sending Des down into the cave just like Smokey wanted.

Des' purpose all this time was to go down there to pull it out resulting in the island shaking a bit.

Then in a mess of a scene we have Jack making Hurley the new Protector who is an imbecile--and Jack didn't even perform the transfer ceremony correctly.
There was absolutely no tension in the 2.5 hours--it just seemed to drag on and on and on and on... Rarely did I ever look at the clock as I watched an episode of LOST but here I did frequently.
There were a few isolated character moments that were pretty good with the standouts being Juliet & Sawyer reuniting and the whole final act involving what the FSW were(which in hindsight makes all the FSWs contrived and nothing more than red herrings) and Jack's powerful realization. However, those few scenes weren't nearly enough to salvage the rest of the hour.
Normally a poor final season or series finale can be overlooked without negatively impacting the overall opinion of a series but not here with LOST's complex and overly interconnected nature. This season was wasted and felt like the writers were spinning their wheels and going through the motions.
This wasn't the worst series finale I have ever seen but it is nowhere near as good as the true gems like TNG's "All Good Things", DS9's "What You Leave Behind", BSG's "Daybreak"--a show I had problems with and who botched their mythology too but Moore did manage to put together a satisfying and emotional swan song something LOST didn't.
LOST is still one of the better series--sff or otherwise--to come out of the last decade but it could have been so much more with a much more focused final season that did justice to the entire series.