Dan, look at what your own definition means...
"...other than being able to cross oceans what's the dif"?Here are a few things that pop into my mind using your definition:1. Weather on the passage must be assumed to cover the full range of types and strengths for that season; convective, frontal, ITCZ-like; normal to extreme - one can't assume less. So...the boat has to accept severe racking and torquing without damage, the sails and hardware have to take the (relentless) shock loading, gear below has to be safely stored, and so forth2. The crew has to be well cared for under these same conditions for an extended time period; often, no marina slip at the end of each passage with a hardware store around the corner, either. Cooking, sleeping, navigating, cleaning and 'ablutions' all must be accommodated, for an extended period, while in a washing machine. Try to imagine something as simple as using a chart table on most of the boats <38' while the boats heaving, on its ear and trying to do the same thing to you. Folks often bulk up on crew when going offshore, for several reasons; is there also room for them? and their gear?3. Substantial load carrying is required, not just because the crew will eat (we hope) and drink, but because of the add'l stores which will still needed after arrival, because ample water supplies in multiple tanks occupies substantial cubic volume, because range under engine and therefore fuel tankage becomes more important (plus a surplus, again because supplies at the other end may be sparse), because an array of spares must have a home, because the boat is probably (over)loaded with systems (electronics, electrical + manual back-ups, sun/wind protection in/around the cockpit, and much more), because the chart inventory (don't forget the guides, pilots, ref books) are heavy and space-consuming...and we could go on. Moreover, all this loading is not supposed to substantially change the ability of the boat to perform NOR affect its heavy weather stability.Folks have done circles in all kinds of smaller, inexpensive boats not intended for a single ocean crossing, so your question is a fair one. But in such cases, the voyages have usually been done *despite* the boat's ability to accommodate needs like the above - and more importantly, because the crew self-sacrificed their comforts, were careful about their seasons and routing choices, because they were lucky and most especially because they were good seamen.Jack