As of the latest Reporting release the JavaScript size to download went from 61K to over 2MB. Our users work for charities and often have slow network connections. This is not an acceptable change for us.

Add the ability to control the relative position of text in relation to the image for both Radio & Check Boxes, such as TextImageRelation property used on your WinForms Radio & Check Boxes.
This would allow the following options: Overlay, ImageAboveText, TextAboveImage, ImageBeforeText, & TextBeforeImage.

On opening the Expression Editor, if at least one Data Source is present on the Report, then the Fields item in the tree should automatically be selected. If no Data Sources are present on the Report, then the default selection should be the Report Parameters.

The ability to use static values and display members as an array for a multi value parameter. I want my team to be able to use the standalone report designer without a custom User Function or a data source. We had been using SSRS and made a switch to Telerik, and this seems to be the only hangup that we are unable to convert cleanly.

Allow custom buttons and other controls to be added to the existing ReportViewer Control Toolbar.

The Standalone Designer allows a template to be loaded from a .NET dll. Add the ability to load a template from a .NET exe.

The report Parameter as of now can only be Boolean, Date, Int, Float or String. It would be great to be able to have a Guid parameter. This is particularly useful when passing a Guid to a child report parameter that in turns needs to be a Guid again to be run in the query. As of today we must convert the Master Guid to String to be able to assign it to a parameter then the child report reconverts it into a Guid to be passed to the query

As a reports author, I would like to reuse one and the same page section in multiple reports, so that I can modify the header at once and spare the copy/paste operation.

As a developer integrating TRDX/TRDP reports into my app, I would like to have built-in functionality to pass runtime data sources corresponding to the data sources defined at design-time along with the ReportSource, so that I can feed the report with custom data easily instead of having to deserialize the report and traverse all DataItem-s

It would be nice if the parameter window to enter Report parameters in  preview mode would remember the last values entered between invocations.

When configuring a datasource through the wizard, the 'Select Statement' TextBox should standard support keyboard shortcut ctr+A to selelct all. Having to open the context menu is a bit awkward.

When selecting several items, it should be possible to change the size and location of all of them, at least if the values are the same

When you have your .trdx and external stylesheet files in a separate directory (e. g. a server directory available to all workplaces) it gets very difficult to show the .trdx files in ReportViewer without manipulating the reports a lot by code.

1. The external stylesheets are searched in the bin\Debug directory of the main application when using Kind="Relative".
2. What is even more annoying is the fact that all SUBREPORTS are also searched in the bin\Debug of the main application. I have to recursively look through all report items if there is a subreport and set its location.

It would be very nice if the ReportViewer supports some kind of "path-to-reports" property and locate all the dependend files like subreport or stylesheet in this directory.

At present one cannot have a subreport in report header/footer. This is such a limitation 
e.g. you have a company header which you want in every report. Normally what you expect is to make a subreport and include that in header of other reports. So if you change any details you can edit that subreport and it get reflected in all other reports. But unfortunately you have to include the company header in every report not only the controls but also in the datasource.
I used to develop in MS Access back in 90s and even then its reporting has this feature as standard.
So come on Telerik 

Create your own parameter syntex, and translate it to the database specific one according to the provider name.
For example, MsSql uses "@" character while Oracle uses ":" to identify parameters.
We need to be able to execute the same report against multi database types.

Currently the list of marker is limited, but it isn't the case for the WPF RadChartView.
RadChartView permits the display of specifics points (like a vertical line for displaying a limit), but it isn't possible in report.
WPF DataTemplate would surely impossible to reuse in report, but some other custome template should be available like SVG, bitmap, or UTF-8 character (not the best customable solution, but it already increase a lot the possibilities)

Ability to make a column in table or crosstab having width based on its content

When converting Crystal Report templates (rpt files) to Telerik Reporter templates it would be very helpful to include the schema settings in the associated ttx file which provides the dataset info. This allows the layout to map properly to the converted template. Also any imbedded SQL statements found in the Crystal template could be captured and the conversion wizard could then query the developer on database connection requirements. These two abilities would make conversion a complete tool allowing rapid deployment of new Telerik report templates.

Change Currentpage, TotalPages, DocumentMap properties to Dependency Property... to enable DataBinding... 

The date format of the report parameters is related to the culture, but the data source parameter is not.