Trawling through related links, I see there's also a set of documented keys like [class:bicycle:...][1] that appears to allow for documenting of the kind of subjective opinion of the suitability of the trail that you appear to be seeking?
Edit [after first comments]:
On further reflection, I see that you mentioned "steps" as being one reason that it's unsuitable for bikes. tagging those properly (highway=steps) if they're not already would surely act as a strong enough discouragement on its own for anyone thinking of riding a bike there... especially if the smoothness, surface, etc. of the other sections of that path aren't that great.
In terms of detail of which secondary keys to use before resorting to class:bicycle, I really have to defer to more experienced mappers. However, there seems to be a good list of keys that routing software for bikes should take into account at [a later section of the class:bicycle page][2], which should allow for ample scope for describing differences between the surface, etc. of the "nice" paths compared to the "nasty" ones.
[1]: http://wiki.openstreetmap.org/wiki/Key:class:bicyclehttp://wiki.openstreetmap.org/wiki/Key:class:bicycle
[2]: http://wiki.openstreetmap.org/wiki/Class:bicycle#Keys_that_every_Autorouting_Program_should_consider_BEFORE_or_IN_ADDITION_to_class:bicycle
