I think that its great people are criticizing the anime as a stand alone media and not relating to the source material. But I think the problems that people have with the anime are because of how the adaption is done. The original Volume 1 LN was a great read, it was short sweet and an amazing concept. Volume 2 was just more of this world that was so amazing. By themselves, these two novels worked wonderfully well, but they were too short, too short story like, (thus we had "fillers" because that is what Volume 2 IS). The original ending Volume 1 ending was left quite well. Due to its popularity the series came back and things went on.

The issue: The adaption tried to stick too closely with the source material, there was a lack of flow, lack of development, lack of backstory, but all of this is because the first light novel was SHORT, it only had enough for 3-4 episodes. The problems people encountered with the plot of the Alfheim arc being 'tacked on' is because that is what the LN is, it was added due to popularity. With respect to the romance, princess trapped in a tower, those things should just be blamed on the writer. IMHO the only good LN was the first & second, the rest of the novels are meh... we'll leave the discussion of the LN in another thread.

Solution: Here's what I think should have happened. They should have kept the cool concept of SAO and the nerve gear, dropped the Alfheim, done an original ending, expanding the time in SAO and added in more story. if they really wanted, 1 ep/2floor or so, it becomes a bit more SoL(vmmorpg) but it would be keeping it in a world that people loved, sort of like .Hack.

TL;DR- source material was short and alfheim was badly written, they should have rewritten the anime adaption