Conceptually, the D5100 perfectly fills the gap in Nikon's line-up between the entry-level D3100 and the much more enthusiast-orientated D7000 - marrying the ease-of-use of the former with the image quality of the latter.
Excellent image quality; Unintimidating interface; but with plenty of manual control; Articulated; high-res LCD screen; Sophisticated AF system for the price; Easy manual selection of off-center AF points; Unusually fast Live View AF for a low-end DSLR (but still relatively slow compared to...
Sub-optimal placement of some second-tier controls (like live view switch; Movie shooting button 'orphaned' from live view switch; Buggy Live View / Movie Mode (movies aren't necessarily recorded at set aperture; No live histogram in live view; AF still a little sluggish in live view mode; full-time...