I will start:

Missing a session configuration setting for timeouts.  What is the current inactivity timeout?  Don't know.  Can you set it?  I do not know how.  iManager had a setting for this.

This is amazingly painful when working on a problem and constantly getting kicked out as you move away to other windows.

Context-less login - Not sure if iManager was this configurable, but we need the ability to manage the LDAP filter in use.  Imagine you have an admin user, an admin group, and admin RBAC role.  Contextless will fail since it finds three objects.  But if we could modify the LDAP filter to what we want (like adding an objectclass check, or even something else, not defined by OT, but defined by us) then we could better control access and contextless.

LDAP Plugin - On the LDAP Server page there is no Refresh LDAP server option. Nor does it do it when you save the changes.  You have to go into ndstrace and unload nldap and then load nldap to refresh it. 

Likely this is because that is an NCP action and Angular supports only TCP and REST so I get it. Likely this will need an LDAP Extension added in an eDir patch, like were done to allow the IDM actions via LDAP. And is non-trivial. Nonetheless it is needed.

I do also understand there is an enhancement request for this one already entered, but worth calling out for people to vote on.

IDM Plugins - There are several deficiencies in these. Probably the worst state of the bunch. Biggest one is missing Policy Builder GUI.  When you look at a Policy in IDConsole, you just get the raw XML.  This is a giant missing feature.  I know this is on the roadmap, but this is something needs to be hurried up, even if it is hard. It really makes IDConsole of very minimal use for IDM. Sure we have Designer, but this is meant to be an replacement for iManager.

Default tree setting - it is a pain to retype the tree name but it has to match the certificate SAN data, so often you have to type the full DN. I discussed workarounds in this article:
IDConsole - adding a default tree 

Short version: Edit the login.html file and you can edit the HTML that defines the login box.

But still this is a workaround since you have to re-do this every time you update.

Try using "SessionIdleTimeout=<seconds>" in edirapi.conf - I am not sure if it works with OSP.

Even in the docs:

Configuring Session-timeout for OpenText Identity Console - OpenText Identity Console CE 24.4 (v1.9) Administration Guide

Missing the option to add custom or aux attributes to groups in IDC. Kind of erratic behaviour depending on what kind of object class you are editing.

The ability to do advanced search when browsing objects.
If I want to migrate all users where company=legal I can do an advanced search in iManager, but I can only browse or search on CN.

I do not know if it belongs to "missing" features, but when you select to use dot(.) it will always end the DN with '.<treename>'. I tried to argue the case and was told that it would being looked at.

Ability to saved advanced searches for future reuse.

In iManager one could define a rather complex search and then save it for later use.
This was exceptionally useful for say a helpdesk or support type role where specific saved searches ensured that the team all operated using the same search criteria.

If implemented correctly, this could also indirectly address deficiencies like RE: IDConsole feature parity - what is missing?  - if the migrate dialog allowed one to reuse existing saved searches for example.