I wasn't even going to register at this forum but when I read this thread, I just had to dive in! My initial reaction to this whole topic is that it's always hard to compare the size of the vocabulary of one language with that of another. Some languages express differences in meaning by modifying some kind of root word (attaching prefixes or suffixes to it, for instance) whereas others use additional words to modify the meaning of the intended message. How many words (or signs) it takes to convey a message is a reflection of how that particular language relays information, but is not a reflection of its complexity.
As for how many words/signs are in the entire language, English is not a very good basis for comparison as it has an almost ridiculous number of words. I read somewhere that a reasonably well-educated native English-speaker understands some 60 000 words, and regularly uses around 10 000. I'll try to find the correct source for those numbers if anyone is interested.
I took a year of ASL when I was an exchange student in the U.S. six years ago. While I was at the top of my class I am FAR from fluent and really don't know enough say whether or not ASL is a "limited" language, but that was never my experience. I would even say that it would be unlikely for a natural language to be limited in that sense since a language that couldn't meet the needs of its speakers would certainly continue to develop until it did, or else die out.
I think different languages have different kinds of complexity though. For instance, my native language, Swedish, doesn't use the progressive tense (corresponding to the English verb ending -ing) and does perfectly well without it. I use the progressive tense in English all the time without missing it at all in Swedish. We simply express that concept in other ways. Conversely, Swedish makes distinctions between many concepts where English has only one word, such as "peace" (where we distinguish between the meaning of "opposite of war" and "a sense of calm") and "know" (there's a difference between knowing a person and knowing a subject). We also distinguish between a grandparent or aunt/uncle depending on whether they are maternal or paternal.
So, where am I going with this seemingly pointless Swedish lesson? Well, one could easily, based on these examples, say that English is limited compared to Swedish because it uses only one word where Swedish uses several. This would, of course, be untrue, but that is the point I'm trying to make. It is nearly impossible to compare the complexity of different languages, and measuring the size of the vocabulary is probably not the right way to do it. Someone already pointed out that classifiers are absent from most ASL dictionaries and that would be one example of a source of complexity in ASL that is absent from English (and Swedish for that matter).
Okay, I'm sorry if I put anyone to sleep, I just thought I'd weigh in on the subject!
/Christine, Stockholm (Sweden)
(A hearing Swede who's an English/Swedish translator, former ASL student and current student of Swedish Sign Language)