I fail to see why people bash on the last few books for being gloomy and serious. Yes there were dull parts where they veered off the plot and the romance was bland but with a evil overlord coming back from the dead and all his servants old and new eager to do his bidding, were people really expecting jokes and butterbeer?
I loved Deathly Hallows and Half Blood Prince the most, I liked how unexpectedly characters got killed and the whole rebellion thing that went on. I could feel that there was a war happening in a world that consisted of more than just the main characters. It had a intensity and suspense I couldn't find in the first few books. Our trio had to be serious now, or they would die horribly. In Deathly Hallows they screwed up and got captured as deserved, eventually to be rescued but at the cost of someone dear to Harry being murdered.
I liked how the characters developed in the last two books. They felt human and relatable, doing what they needed to survive like any of us would have done in their shoes. They also reacted like how people in their current mentality would, which while hard to read sometimes just served to show how bad things were going.
Life is not smooth sailing and I know literature doesn't have to reflect reality but if they got through the whole thing laughing and messing around wouldn't that just be cheap and too unrealistic?