yeah, built-in shake reduction is huge. it doesnt do anything for moving objects with slow shutter speeds. but
all your lenses are IS/VR/OIS.
now i type the rest of this having started with the first consumer digital SLR, then went to a mid-level camera, and now have a "prosumer" one...
the thing about entry level cameras is the viewfinder is too dim to manually focus...but they always have fewer AF points and less accurate focus than a higher-model camera
big and bright matters. its the difference between eyes in focus or "hmm, i guess the eyes werent in focus now that i cant retake the picture..."
Raw quality is pretty much dependent on the sensor itself. some can take 14-bit images, others only 12. you wont notice a ton of difference until you print in foot by foot sizes
high ISO handling, though, is essential. older cameras just cant keep up. ISO 400 max is seriously limiting and frustrating. the takeaway from this paragraph is be prepared to do some post processing...
shutter lag is non-existent and manual controls are pretty universal. what you will notice, however, is how different brands handle manual mode. with pentax cameras, you can use digital preview after pressing the green button to see if your exposure is close. the others, you need to guess, take the picture, then adjust exposure. some cameras have focus confirm without a modern AF lens hooked to it, most dont.
so a quick rundown on lens compatibility if you want to use legacy lenses...
-canon...lots of brands and mounts can be used with adapters. no focus confirm without an adapter with a chip (somewhat flaky). pretty much M and Av modes only.
-nikon...usable with most nikkor lenses made since like the early 70s. only nikon (and lenses made for the F mount) lenses reach infinity focus.
-pentax...every K mount ever made, M42 and M39 with adapter(s). focus confirm, shake reduction, digital preview, auto aperture.
of course i'm a fan of pentax because it has the most value for the features. i would take a look at a K-7...i saw one posted on pentaxforums for $385. from there, the sky is the limit for lenses.