First of all is the price: the NEX 5 isn't cheap. The least expensive package (the NEX 5 camera along with a 16mm f/2. 8 “pancake” lens) is actually $650. For $150 more, you can get a kit with an additional zoom lens (18-55mm f/3. 5-5. 6 image stabilized) along with a small flash. But at a cost between $650 and $799, the NEX 5 is actually clearly in DSLR-pricing territory. For many people, this means choosing between the NEX along with a Canon T2i or a Nikon D3100. Sony has a NEX 3 for $549, however, you will give up the metal body and higher-resolution video capacity for the 5.
The next hit towards the NEX 5 is also one associated with its advantages: its size. Sure, using the 16mm f/2. 8 pancake lens connected, the NEX 5 is fairly svelte and is convenient to carry in a bag or pocket. Attach a different lens though (like the 18-55mm) and any thoughts of putting this camera inside your pocket will immediately melt away. May be the camera still smaller and lighter than the usual DSLR? Totally. But if the size means you need to carry something around in an extra bag or on the camera strap anyway, why not simply grab your DSLR instead?
Another issue may be the lens selection. The NEX 5 runs on the new lens mount (the E mount) and also the pool of lenses designed for it's pretty sparse. Sony only has three possibilities right now: the 16mm f/2. 8, a good 18-55mm f/3. 5-5. 6 and a good 18-200mm f/3. 5-6. 3. Granted, this can be a new camera system and it takes time for lenses to become developed, but that doesn’t change the truth that DSLR’s from Canon, Nikon, Pentax, and so on. all have dozens of lenses obtainable already. Even the similarly sized Micro 4/3rds digital cameras from Olympus and Panasonic have more lens options available on the market.
My final complaint is with the actual controls (or lack thereof). I actually do appreciate the sparse, minimalist design from the camera, but I quickly find myself missing dedicated controls for such things as ISO, WB, Focus area, etc. Most abundant in recent firmware installed, some of these things can be assigned to among the 2 custom buttons on the digital camera, but it still feels very restricted. And even after months of make use of, I still can’t figure out a method to lock exposure on a subject as well as recompose the image. This camera feels more like the point-and-shoot in these respects and you’ll need to be willing to give up some degree of control while using it.