[batbomb]

You're just trading one arbitrary thing for another, IMO, but what's worse is you are now abusing the URL specification for the HTTP(S) protocol, so nobody can use existing HTTP URL libraries.

You could easily say everything before either ? or ; always refers to a database, and use a query parameter or a semicolon to delineate a dataset. Or you resource paths:

Address a dataset:

    http://demo.noms.io/?dataset=cli-tour/sf-fire-inspections/raw
    http://demo.noms.io/;cli-tour/sf-fire-inspections/raw
    http://demo.noms.io/dataset/cli-tour/sf-fire-inspections/raw

Address database (catalog):

    http://demo.noms.io/database/cli-tour/sf-fire-inspections
    http://demo.noms.io/catalog/cli-tour/sf-fire-inspections

Address dataset in that database:

    http://demo.noms.io/database/cli-tour/sf-fire-inspections;raw
    http://demo.noms.io/database/cli-tour/sf-fire-inspections?dataset=raw