I'd add to @mocacinno pretty good list:
* [very important] support top hardware wallets (at least Ledger and Trezor)
* [important] the seed should work with other wallets too
* [important] it should be SPV since most users don't have the patience to download the blockchain
* [less important] allow RBF easily
* [less important] show the amount also in fiat (at least USD) <- I find this very useful for newbies that could get confused about how much money they send

The discussion on UI can be wide, but I don't want to write so much. I'd modify this on that on Electrum's UI and behavior (I'd go back to the old handling of SpendFrom, I'd default to BTC not mBTC, ...) and I'd add an option for simpler view for newbies.
All in all I do like Electrum for PC, but mostly the older version. So I'd start with that, I'd change a little here and there and I'll try to find ways to make it less confusing for starters.