I just finished "Best Short Novels of 2006" edited by Jonathan Strahan. (This is specifically within the speculative genre.) I enjoyed reading novellas - a form I don't encounter a lot - and had some interesting observations, some positive and some disturbing.
Disclaimer: next two paragraphs are specific to fantasy. The anthology was 50 - 50, so this applies about partway.
One thing that I was genuinely surprised (and somewhat disappointed) by was the fact that the majority of the stories were grounded in an Terran environment - contemporary earth, future earth, alternate earth, but still essentially a world playing by most of the rules of our own. To me, one of the hardest parts of short fiction is inscribing a fantasy world in a few brief pages, so I expected novellas to be a place where exotic worldbuilding could shine. The closest there was to a fully-realized secondary world setting was the Library embedded within Kelly Link's "Magic For Beginners." That said, however, many of these novellas took the time for leisurely tidbits of setting that didn't necessarily hinge upon the plot - and that was very enjoyable.
The other thing that made me squint a bit was the paucity of magic in the stories. Gone are sorcerers and systems of magic. Those stories that seemed to deal closest with supernatural tropes either presented them with technological explanations and trappings, hid them in an in-story television show, or clothed them within a child's voice. Maybe it's just an accident of the novellas printed in 2006, but you'd almost say these writers were allergic to it. This says nothing about the overall quality of the stories, of course, but I found it a weird trend.
What the novellas here seemed to do best was give a sense of history. The stories had room to expand and travel through several periods of the characters' lives. Some were less plots as biographies. It was interesting to have the chance to explore events outside a strict plot itinerary ... and still feel, as a reader, that you were going somewhere.
Of these, I would have to say my favorite is Connie Willis' Inside Job. It's an intelligent, crackling, hilarious story - and it does a good job of sprinkling the author's research into the tale without ever being obtrusive. This is also a story for anyone who has ever rolled their eyes at New Age money-making schemes.
I was disappointed by Harry Turtledove's Aubudon in Atlantis. It had a great premise - our historical Aubudon searching for vanishing wildlife to paint in an alternate reality where Atlantis was its own continent - but the story itself was very dry. I knew I should have been invested in the characters and their quest, and I just couldn't get there.