So I just learned today that Marketo only synchs one activity of each type per day to Salesforce. (E.g., if one of your leads receives two emails in one day, only one of them will end up in a Salesforce activity).
This creates a massive hole in terms of reporting and sales visibility.
The whole idea behind CRM is to have all your interactions with a customer in one place. If only one activity per type is synched per day, then any subsequent activities will be "lost".
Sure they are still in Marketo but Sales people will not have that visibility, which in our org they rely upon.
Furthermore, we use Salesforce activities as the basis for some of our reports on emails (this is how we've structured our data warehouse) so this means that some of that data is unreliable. (Gulp!)
Anyhow, I feel like this is a critical gap and would love to see the option to synch all activities to Salesforce, regardless of whether it's the first email or the 10th email of the day. Whatever happens is what we should see in Salesforce. Personally, I don't see the logic of doing it any other way (why capture only some of your data?)