The only quirk I see, as opposed to multi-touch (iPhone) is that you'd have to set this device down or hold it in very awkward ways. If you're at a desk that's fine, but it'd lead to precarious situations with not holding on very securely to your device.
Other features that have piqued my interest (besides the already mentioned, and vitally important, macros) are:
- conditoins and restarts
- CLOS
- reader macros (which are distinctly different from normal macros).
If you're talking about other Lisps (besides CL), like Scheme, it's worth looking at define-syntax and syntax-rules and how they deal with some of the intricacies of creating macros with define-macro.