By increasing it to 30 witnesses you add a few areas of concern. But I first want to be clear, we do not have mining.

Confirmation times go up dramatically, right now we have roughly 51 seconds to confirm a transaction, now most people use the head block and only wait 1 block (3 seconds) but any transaction in theory can be rolled back before the super majority confirm it. This is called an irreversible block, but not every system waits this long, most don't. With 30 witnesses, this will be longer.

You also need to have more people physically available when doing a hardfork or emergency patch, this is can be a bottleneck in critical times.

Most things won't change at all, as most things are using small transfers, they will not wait for an irreversible block.