I really liked Lost at first, then stopped around mid-way Season 3....then picked it back up again until the finale. What frustrated me about the show is that they set up all sorts of mysteries, then kept building up more mysteries. The solutions and reveals were increasingly unsatisfying. Most frustrating, though, is the way they presented the mysteries, it felt like they knew exactly what they were doing...but they didn't. It became increasingly clear that they made it up as they went along and that's just not how it should've worked. If it was planned ahead like Babylon 5, it could've been great.
As for Dexter, I partly agree with Charlie: it's very hit and miss. Sometimes you'll get a fantastic season like 2 or 4. The rest is watchable, but barely. What pisses me off isn't just how less likeable Dexter becomes as the show progresses because he's crossing more lines coke addict. It's also that the cops he works with look progressively more like fucking morons. I not only assumed last season would end with him locked up (because it was a natural progression and made sense going into the last season). But no, no of course not. And now we're in the last season and I get the sinking feeling this monster will get a happy ending because he's the main character.