Lauren started off as an irritating roadblock, a plot device designed merely to keep Syd and Vaughn apart. She wasn't a bad character but she wasn't all that likable or interesting. Until they revealed her to be bad. Then she became 500% more fascinating and fun as a character. I agree that it was unfortunate that she was killed off at the end of the season.
Season 3 was a bit uneven, but I don't think it was a terrible season. It just wasn't as tightly written or cohesive as the first two seasons. It introduced some interesting plot points and mysteries but poorly resolved/developed them. Basically, it was an adequate premise but a disappointing execution. The mystery surrounding Syd's missing two years was cool, but the reveal was ultimately underwhelming. The introduction of the Passenger was fine, but making her Syd's long-lost half sister was lame. I also feel like they did not adequately follow up on Nadia's role as the Passenger in subsequent seasons.
Season 4 is a massive disappointment from beginning to end. There's very little to enjoy. Season 5 starts to get back on track and I really like the season, but at this point it's too late and the show gets a rushed, poorly-planned finale that I was not happy with. Ultimately, I find Alias to be a really fun, entertaining show with great characters, but the plots and storylines are horribly contrived and poorly executed (at least, everything after season 2).