Well, there are other problems (even requiring encryption at all means some low-power clients can't use it because they lack the processing power), but X.509/We

Well, there are other problems (even requiring encryption at all means some low-power clients can't use it because they lack the processing power), but X.509/WebPKI is the main one, followed by ciphersuite proliferation requiring dozens of times as much audited cryptographic code.