There was a suggestion earlier in the thread that Stephen Hawking thought that antimatter might provide the key to feasible interstellar travel. No link provided, though he would probably have mentioned it in one of his books -- The Universe in a Nutshell, perhaps? (Which, for what it's worth, is the book that inspired me to take up Science as a career path in the first place). But even then it's still highly speculative and while it might lead somewhere and is certainly worth exploring, the amount of antimatter required could be just as prohibitive as any other limit. Depends a bit on the type of antimatter, but on the face of it you're looking at an amount measured in kilograms, which isn't much on the face of it, but again compared to today's efforts of something like only a few hundred atoms of antihydrogen lasting for a few minutes, we'd need to find a way of producing about 10^25 times as much, and making it stable, so you're still faced with a huge task. The was a suggestion that, at current rates, it would take us 100 billion years to produce just a gram of antihydrogen.
Whether or not these challenges can be overcome remains to be seen, to be sure, but they are certainly vastly greater -- almost unimaginably so -- than anything we've managed to achieve so far. It's important to recognise this, but even if it's impossible it probably wouldn't hurt much to try (although that said, given that the energy needed is equivalent to the resources of an entire planet, then it's more likely to be something to try if you're not intending to come back).