Series tend to end in general one of two ways, it was crap to start with and just ended badly, it was great to start with, got loads of seasons, got worse every season and was so crap by the end it was a crap ending to go with the general quality of the show.
Friends was fantastic for 3, maybe 4 series, went downhill significantly was truly awful in the last couple seasons and a crap feel good ending with crap jokes, crap lazy acting and was just crap.... ok Ross was always good, everyone else gave up half way through the show. Then you've got something like SG:U with a mostly awful cast, mostly boring episodes, sure it improved towards the end but it was still ultimately a very weak show and a meh ending.
The few exceptions are something like Terriers, which was fantastic start to finish and ended, quite like Veronica Mars, with an overall season story arc ending well... but the show was writen with an ending that left it ridiculously open for a new series... and both those shows should have gotten them.
Ultimately shows designed with general season long arcs, like Veronica Mars or Terriers, Buffy, tend to finish quite well with the option to stop or do a new series. Those with multi year long arcs (Bab 5, DS9) finished where essentially they had to, with the end to the main story behind the entire show, with each season having its own arc usually.
Worst thing for shows is network indecisiveness, either giving a show 13 eps, it builds towards a good finish or good season end, then a network extends it... and you get a ridiculous second half to a season, Prison Break was one like that. The other way around is half way through filming a season a network decides to can a show, and suddenly a full season arc gets cut short, and messed around to fit in an ending to the show and that usually ends up crap.
