I've recently read new Verbitsky's novel "The chronicles of Zarechensk" (Хроники Зареченска). The novel was rather good, but I was disappointed that author had decided to make sequel. And this sequel is hinted in first part: some new plot lines are created and "to be continued" is written in the end with bold font.
So, I tried to remember different interesting (or even outstanding) stories and their continuations. Let's take, for example, Lukianenko's "Night Watch": great book, interesting plot, and this books looks finished. But there are sequels "Day Watch", "Twilight Watch" and others. Some of them are very good too. I like several extracts of "Day Watch" very much. But the novelty is already lost, other book are not so thrilling. Let's take Perumov's "Godsdoom". It was exciting. This book gave background description of the whole universe. And later he spoiled impression describing this universe in all details (I think in about ten books :)). In this sense, the perfect example is "The Lord of the Rings". It's really cool that author had written "Silmarillion" for himself and didn't publish it. So, the whole book has fascinating background. It is filled with songs, legends, smell of the former epochs. A lot of facts and plot lines are just mentioned and not over-emphasized. But this is a complete universe. It is described in several books and it is whole. On the other hand we have bright "Dune". It is sometimes called the best sci-fi book of 20 century. And it deserves this title. Rich fantasy of Frank Herbert created extensive world with a few traits. Mentats, Bene gesserit witches, powerful clans fighting for Spice... And most of this became quite boring, I was fed up with this when I finished his sixth or seventh book.