Firstly, please don't compare Harry Potter with Twlight, is just painful. Although I have to agree that Harry Potter is an easy read (well, but not as easy as other books) is still a good read whilst Twilight...I have no words for that.
Secondly, I have to agree about modern literature. I'm not a fan of it, read some books though and all of them each in less then a day, and I could say they were quite an easy read - very easy, in my opinion. I'm mostly referring to the writing style which in my opinion I think everybody can write such things - in most of the time is just story-telling; it seems so difficult to transpose some feelings into words nowadays without appealing to the usual "He was so angry...etc" and just giving the reader the piece of information on a plate.
And yes, the subtlety - in most thrillers nowadays, there is none. The usually Byronic-fake cop/detective who speaks like dirt tries to solve a murder or something of that sort, and the book is already packed with hints and clues that the reader already knows who the killer is and is only a matter of time till the cop will found him. This is just a mere example, mind you.
And also it seems that the plots resemble each other in various ways... it seems the term of 'original' no longer defines a good read. Which is a pity. Plots like "girl-meets-boy" like someone here already said, or "the good guy against the bad guy" or the "war between planets" one...
Plus, I've realized by a couple of modern books that I've read that 'seduction' no longer exists in this society - it's either fluff or rape - whilst in books like 'Dangerous Liaisons' the character is clever and knows his abilities.
It's not strange that I'm an avid reader of older literature.