UX - Despite joining their customer UX council (and having very little ability to steer the product team), the usability of Engage needs the most attention. From not officially working in several common browsers - Safari, Chrome, to cryptic notifications and cumbersome processes (manually having to recalculate list totals or going through a confusing process to send an email), the product is just really dated. Old-style drop down menus and annoying in-between pages greet you when logging in. Even the recently updated homepage of data widgets (an attempt to make it feel newer), is really limited in functionality and customization.
Email creation engine - Other ESPs like Bronto and dotMailer (yes, I realize these are different level tools) have advanced Ajax or jQuery based creation engines that allow significant flexibility in layout design without having to hardcode each email. Do you want three stories instead of four or two columns on one row? Easily done in these other tools, but a manual HTML process in Silverpop.
User profile controls - We were never able to isolate users from gaining access to the core database. An inexperienced user could have easily sent an email to the entire database by accident. Also, users were often locked out and were not able to reset their own passwords. The password reset function never worked, so the admin had to handle each one. Additionally, if a user creates a list or query and leaves the company, not even the super admin can edit or delete it.
Reporting - Overall reporting in Silverpop is very weak. Confusingly, there are two different reporting engines, one legacy, one new. You could edit the links after an email was sent in the legacy, but not the new. They were not flexible, and despite hundreds of users asking Silverpop via its customer community to remove Test sends from the list of emails in the reporting tool, they're still there. If you want to see the contact info for opens/clicks to provide to your sales team as an FYI, it's buried on one of the reports.
Forms - Signup forms are part of the 'landing pages' functionality, which is one of the more buggy parts of the product. On several occasions while editing a form, my browser would lockup and I'd have to reopen it. The usability also suffers here. If you want a list to be available to allow people to opt-in, you have to exit the landing pages section, find the list in the Data section, and then buried on a tab is the "allow opt-in" checkbox. There are also significant limitations for the forms including styling, no adoption of existing CSS, and it uses an old school table-based layout.
Auto-reply handling - We all know that despite telling recipients 'please do not reply', people still do. We found no way of reviewing those replies.
Services and Account Management - As a fairly large ESP, we had expected a more comprehensive implementation services offering in-house. We were only offered a quick start program. In the last several years, I can say that I've spoken to the ever revolving list of assigned account managers less than five times.
Asset library - More than a year into using the product did Silverpop finally add the ability to upload and link to PDFs in your emails. The asset library does not provide thumbnails of the images you upload while browsing (only have file names to go on), and it does not offer an image resize tool. Additionally, there is no notification or auto-renaming if your filename contains spaces. Something that they do alert you to only just before you hit the final send. But even then they don't tell you which image has the issue!