I didn't watch the Anime, but I read the Manga (Which probably wouldn't change the story much, but.. who knows.)
The ending was a bit meh, but the story was pretty cool with the turn-arounds.
Light - Flipflops between sheer genius and utter stupidity. He pulls off incredibly elaborate plans to get what he wants done, but which were only necessary because he led the investigation right to himself.
While I agree, there are times where Light didn't exactly understand what was happening. For example, after he lost the Death Note, all his memories of being Kira were erased, and as such, he led the investigation with L to the second owner of the Death Note (That company guy.) Eventually he came down to a corner and got all his memories back, but that was only because he didn't remember that he was Kira until he touched his Death Note again.
Claim that justice must be done within the confines of the law, but then repeatedly break it. They use all kinds of illegal surveillance, intentionally use criminals as test subjects for Kira, and torture suspects (specifically Misa). It's also worth noting that by the time they actually catch Kira, what he's doing is no longer even against the law (as he's been accepted by the government).
On the Manga, the Cameras installed in the Police's Officer's house (A Police Officer, in the manga, is Light/Kira's Father.) were installed by L (the detective.) Let's also not forget that the Police, in the beginning of the MANGA, at least, broke up with Light's Father and the other Officers because they were tracking down Kira, and Kira was too dangerous. After they Broke up with the police, they aren't Officers anymore and they don't need to follow the police conducts. However, they didn't even do anything illegal as everything Illegal (At least on the manga, if memory serves) was provided by L.
By the time they capture Light/Kira, countries accepted him out of fear. Lots of other countries however, were still against L. The country they lived in being one of them.
So the author decides to have Kira kill L, only to introduce a new character that is almost exactly the same (Near) 20 minutes later. It takes away basically possible emotional impact of L's death, and just leaves a feeling that the author just wasn't creative enough to come up with an actually new character.
According to the MANGA, 2 years had passed. On those 2 years, Kira was accepted by countries, while Light had turned into the new L and continued the "Investigation" at Kira. It would be 2 years of Void were Light would lead his father and the others to dead ends.
I wouldn't say that it's bad. I's just say that, compared to what it COULD HAVE BEEN, it's.. a little bit off. It sure has potential for way more, but..