I bought my DSLR when DSLR prices were plunging off a cliff, that is after all the professionals had subsidized sensor and lens development. I bought the D40. I got a DSLR for the following characteristics:
- Low shutter lag. This was probably the biggest deal for me. I like to capture the fleeting expressions on human faces and the compact was very frustrating with the long lag between focusing and then taking the picture.
- Good low light performance. The D40 works just fine for me upto 1600 ISO. ISO 3200 is very noisy and adding a nice prime lens that goes out to f1.8 added a lot of artistic scope and improved low light performance.
This has turned my attention to the micro four thirds family. The larger sensor sizes are a great step up from compacts, but the form factors are so small! They also have interchangeable lenses.
Shutter lag is still a concern, but one thing I realised after using the D40 is that in low light (when a lot of my people portraits are done, round dinner tables and indoors) I have a long effective shutter lag because the focusing in low light is an issue.
What I depend a lot on in such situations is to focus on a sharp edge and then shoot a burst. Instead of waiting for the right moment, I estimate when the moment is going to come up and then hope that one of the images in the burst will carry the hidden expression.
The new 4/3 cameras I am seeing do bursts, do better ISO than the D40, are smaller/lighter AND they do movies, so I'm pretty sure my next camera is not going to be the D5100 (I was waiting for the price to drop steeply, or to find a refurbed one) but rather one of the 4/3s family.
UPDATE: I just found Thom Hogan's guide to m4/3. The guide is very useful.