You should add THE SOPRANOS finale to the other two, because although I thought it was a perfect ending, it pissed off a lot of its fans as well. As far as your original two, I was most definitely "pissed off" at the entire season eight and seven of DEXTER. I had no problem with the SEINFELD finale as it allowed the audience to reminisce over past episodes by re-introducing some of the best guest characters by having a trial as to the ethics of the four main characters.
"Whither is fled the visionary gleam? Where is it now, the glory and the dream?"

Never really watched Seinfeld, so no comment.
Dexter, on the other hand, had an extremely bad finale.
Angel's finale pissed a lot of people off, left it toooooooooo wide open.
Bobby: "You don't shoot Bambi, jacka$$. You shoot Bambi's mother."

I liked Seinfeld and the ending was lukewarm than rage-inducing: the trial was a clip show like the previous episodes (the special, not the Puertorican one) and the whole legal basis doesn't make sense in real life (the gangs aren't officers or something).
I never watched Dexter and I don't care.
How I Met Your Mother was painfully executed after a slow, but steady decayment of the show.
I don't care about endings, as long the whole show is consinstently good, but is very disappointing if it become a bad prize for an endurance test.
Lost was compleatly off the wall and they could avoid to rely on ambiguity once more.
Malcom in The Middle was very stupid and unsatisfying: how could a smart kid really bought the dumbest idea ever in few damn minutes and being cool with that?
I never found the show that funny, despite some sparks here and there, but a last middle finger to that jackass of a family would've been cool.
According to Jim was out of the place with the heaven trial thing or whatever. It doesn't offer any form of conlcusion than the previous episodes (or seasons, debatebly).
Scrubs was fine with Season 8, but Season 9 was a failed effort for a spin-off and it ended anti-climaticlly, like if another season was a certainty.
"Its Not the Idea But the Execution that Matters"

My Name is Earl.
~
Not my circus, not my monkeys
~

I didnt mind the end of seinfeld
Never made it to the end of dexter or lost
One finale that pissed me off was metalacalypse

I don't care about series finales.
I liked Seinfeld.
Dexter didn't interest me. I can't get any jollies out of seeing anyone be murdered.

I am different than most, neither of these. I think the worst one was Cheers - bringing back the boring dumb blonde and putting the bar back to episode 1 basically was one of the dumbest things ever done.
Shelley Long got annoying on the series long before this swan song and if she had stayed it would have ended many years sooner. Cheers proved it ran too long with this dud finale.
A Smile is priceless and Can Be Addicting

I think Dexter, which was loathed unanimously. The Seinfeld finale was defended by some, even I thought it was not horribly bad. I think just the tone did not fit, I never thought the Seinfeld characters were so terrible persons that they had to be brought to justice by the world.

Nothing was worse than the monumental cheat of Lost's alleged Big Finale.
And I didn't think the Seinfeld one was badit simply underscored what the premise of the series had been all along: 'a show about nothing'.
The finale was about nothing, and was totally in keeping with that.
50 Is The New Cutoff Age.

I haven't seen "Lost".