1script - 11:29 pm on Apr 27, 2013 (gmt 0)
Yes, I would be very cautious with even having View All option available to users, let alone Googlebot and thousands of other crawling creatures that will hit it. I don't know about 1000s - I tend to split those up - but 200 or 300 responses in a thread slow it down significantly and 500 in one thread could bring the server to its knees - had this happened before. And what's sort of ironic - as soon as a troublesome page like that appears on the site, Googlebot starts to hammer on this page and make the load / speed situation site-wide or even server-wide worse.
problem with a view all with forums is some forums have lots of 1000+ post threads
Personally, I would not change pagination on a popular site. I did it before and ended up with a ton of additional 404s because people were bookmarking pages of the treads that were eliminated because the posts were packed more tightly. Say, a 100-post thread spanned 10 pages before. You made it 20 per page and now pages 6-10 are 404. Your natural reaction would be to 301 them [somewhere] and that in itself might be troublesome. I think they don't like a page receiving too many redirects from other pages. So, if you 301 to the first page of the thread - its title page for all intents and purposes, the first page gets penalized (or algorithmized - call it whatever you want but it has trouble ranking). Since the first page of a thread will almost always rank higher than the rest, the entire thread gets fewer visits, and that's automatically 4 more pages will less traffic.
I've been in this rut for almost 2 years now (although other changes were made at the time, too), so don't make that mistake, at least not without carefully considering where you'll send the traffic from the pages that were "compacted". Or decide that you will 404 them - also a valid option because at least in theory the thread can pick up more posts and become 10 pages again, only now with 200 posts.