If you want to have scripts like "show more" without starting the video downloading and you're using NoScript, the simplest solution is to go into
NoScript Options > Embeddings and check "Apply these restrictions to whitelisted sites too".
That'll get you FlashBlock-like behaviour (which is designed to be a secure protection against Flash exploits, unlike FlashBlock which is for annoyance-reduction) even on sites which you've marked as trusted.
The only downside is that NoScript for non-mobile Firefox doesn't yet have the extended settings support to allow you to set or unset "Apply these restrictions to whitelisted sites too" on a per-site basis, so you'll get FlashBlock-like behaviour everywhere and you can't whitelist any sites.
The alternative would be to allow
ytimg.com and then install Greasemonkey and a script like YousableTubeFix which lets you set "Prevent both autoplay and autobuffering".