An ability to crop exact pixel dimensions is something really needed, as it frustrating trying to get 2400x3000 exactly. For the lab I use for prints, I know they print at 300dpi, so I crop accordingly...LR3 Beta means you can only do an approximate crop and then have to finish it off elsewhere.
This ability is also missing in ACR in CS3 (not yet moved up to CS4), yet Canon has had the ability to specify pixel dimensions for a specific aspect ratio for years in its DPP program. At least in ACR CS3, I can zoom in to 100% or more to make it easier to get exact dimensions...tedious and time consuming in ACR 4.6, but it works. However, even that option is not available in LR3 Beta.
So, suggestion #2: allow full zoom control to 100% (and higher) when using the crop tool.
Suggestion #3: In ACR 4.6, the image size and ppi are displayed underneath the image, so you do not have to turn on an obtrusive info output to see image dimensions as in LR 3...needs to change LR. Also, ACR is easy to change the resolution (e.gg. from 240ppi to 300ppi).