I'm pointing to ridicule situation - there are thousands of unconfirmed transaction but block will be accepted, which means
blockchain size will be increased by 1, even though mentioned block has no transactions other than 50BTCs payout. Since
that is possible, it's possible for determined attacker to "freeze" all non-BTC-generating transactions (almost) indefinitely.
Not really. There are actually several ways to counter this if we ever need to.
Not too long ago, there were a lot more of these no-transaction blocks and there was speculation that it might be a botnet doing them. If it is a botnet, the rise of ASICs over the next few months will pretty well kill them off. If it isn't a botnet, we can make it costly to be antisocial if we really need to.