What professional features does an XTi lack that you find useful?
The XTi is a very complete camera, and totally usable professionally. It is so nice to see all these threads just being rants from people who have to defend an earlier purchase, even if that purchase was done without all too much knowledge.
To make an analogy, a pro cyclist with a cheap mountain bike could out-race most regular people on expensive carbon-fiber bikes - being a professional photographer is far more about the person than the camera, but they can still get further with the right equipment.
Image quality differences between most newer DSLRs are negligible (unless you're an extreme pixel-peeper), so yes an XTi could be used by a professional - but most would choose a more appropriate model for features like:
- robust build and/or weather-sealing
- bigger/brighter viewfinder
- faster burst speed
- multiple storage card slots
- longer shutter life
- extra weight to balance larger/heavier glass
- faster direct access to features (multiple command dials)
- spot metering, improved/faster focus, higher resolution, etc. (depending on the camera)
The XTi is a fine camera with great image quality (although it's been criticized for ergonomics and some underexposure/sensitivity tradeoffs), but professionals are simply not the target demographic Canon has in mind for it. Use a pro or semi-pro camera professionally, and then go back to an XT or XTi for a paid event where you're under the gun - you'll never need to ask about the differences again.
epicwelshman - having owned both systems, Canon/Nikon are both solid choices, noise/sensitivity is pretty much at par with the latest offerings, pro glass is mostly a wash between them (many think Canon has a telephoto edge, and Nikon has the wide angle), consumer lenses are slightly better with Nikon, a few more lens options with Canon...bottom line is you won't make a mistake with either system. The Pentax K10D looks like a great camera, although I haven't used it personally (there's a joke about non-Canon/Nikon cameras simply taking up shelf space that could be used for Canon/Nikon accessories).
Although I currently use Canon, if I was starting out from scratch today I'd get the D80 (ergonomics, viewfinder, image quality, built-in wireless flash trigger, etc.)...wait a few months and Canon may come out with a 30D successor that betters Nikon, but for today...