I see that one of your favourite books is The Unbearable Lightness of Being. I bought it and got about half-way through, but it didn't really compel me to read on. I will pick it up again as people keep talking about it, but I still do not understand the lure of it. It's possible that the books that I couldn't put down are put-downable by someone else.
For example, I was completely taken by Oracle Nights and The Brooklyn Follies by Paul Auster. I recommended them to a friend and she was turned off/disgusted by his use of the word Chinaman - I think - to describe a man who owned a book store. Whoops -but I feel that's a poor stand-in for truly knowing
of course it's a poor stand-in for being so grabbed by a book that it must be finished ... but if the first 100 or 200 pages don't grab you and it's a slog, I don't see how reading on will give you a feeling of getting why other people were grabbed by it.