First, let me echo Wraith's suggestion that you consider the new form factor of "mirrorless" cameras. They also sport interchangeable lenses, and are far smaller than DSLRs, making it easier to have them with you wherever you go. That said, my approach is tiered: I've got my phone if I just happen to spot something; I've got a ruggedized compact for active stuff, and I've got a DSLR for when I want to do real photography.
Also, two tips on what to look for regardless of the form factor. First, don't get caught up in the megapixel count. Pretty much anything you'll find is going to have more than enough resolution for you. Second, the big leaps in technology in the last couple of years have been in handling low lighting gracefully: today's cameras can deliver clear, sharp pictures at ISO numbers that, a couple of years ago, would have made a noisy mess. So if you decide to save money on a used camera, be sure to evaluate this feature of the camera carefully.
If you decide that a DSLR is really where you want to go, let me suggest a dark horse candidate: the Pentax K-30
Much of the photography debate is monopolized by Canon and Nikon, but there's really no good reason for that. Olympus is still out there; Sony is resurgent (although Sony products aren't allowed in my house, but that's a different story). And Pentax has historically been one of the leading camera and optics makers.
One reason that Pentax gets left out of many discussions is that their line-up doesn't fit neatly into the same slots created by "the big two". In particular, the K-30 is clearly a very strong mid-range contender, but its pricing gives a lot of bang for the buck.
There are a couple of attributes to the Pentax line that make it stand out from other competitors:
- Backwards compatibility - any Pentax SLR lens ever made can be used on current Pentax cameras (in a few cases a cheap adapter is required). That means you can get cheap, good-quality used optics off of Craig's list, or use your dad's old lenses from the closet.
- Sensor-based image stabilization - the image stabilization function is achieved by mounting the sensor itself on a movable platform (Sony does the same). This has a couple of great effects. First, those old backwards-compatible lenses automatically benefit from the image stabilization, and you don't need to pay more for the feature on new lenses you buy. Second, the way it's mounted allows for additional degrees of movement that lens-based stabilization can't achieve.
- Rugged usage - All current Pentax DSLRs are designed to be used in unfriendly conditions (rain, beach, very cold weather). You wouldn't want to take your Canon or Nikon out in the pouring rain, but my Pentax doesn't care (so long as I'm using a water-resistant lens as well)
Of course, there's no such thing as a free lunch. I've found a couple of down sides as well. First, because the Pentax market is smaller, lens manufacturers don't cater to it as much. That means that there's a somewhat smaller selection of available lenses, and they tend to be more expensive (this is greatly mitigated by the fact that you can buy cheaply all that old used glass). Second, also because their market share is smaller, there are fewer people around that you can go to for help, or to test out other products (lenses, flash, etc.).
If you're willing to consider it, I'll leave you with pointers to some reviews. These sites should be good resources for reviews of other cameras as well: