The mempool will likely contain more transactions because runes will likely have an initial novelty/popularity

Blockspace will likely be scarcer due to the greater number of transactions trying to get into the blockchain (hopefully their initial popularity will wear off)

The utxo set is unlikely to increase significantly because runes are encoded as op_returns, which have no impact on the utxo set

To create a rune you must spend at least one utxo, but you need not create any. Most users will probably create a new one to replace their old one with change, so they will consume utxo A, create utxo B, and create an op_return, which is not a utxo -- so, overall, no material impact on the utxo set (hopefully)