For reference, here's a list of the changes that have been made to the SLX Editor (excluding those which deal with SLX-only features).
Most of these changes make a noticeable difference to the overall layout and functionality, which some users may consider to be an improvement:

Widened dropdowns for Theme/Style for easier style browsing

Widened horizontal and vertical scrollbars for easier access

"Auto Screen Start" is no longer checked by default (but its state is remembered when opening a level in the Editor)

Selected pieces can be moved horizontally-only and vertically-only by pressing a hotkey whilst dragging with the mouse

Alt key is assigned to "custom move", which allows the user to specify a number of pixels by which the selected pieces will be moved when pressing Alt + Arrows (this feature effectively brings back the old "Shift Level" functionality from Editor 1.43, whilst also being useful for individual pieces, multi-selected pieces, and grouped pieces)

The Talisman creation dialog now warns the user if no requirements have been set - the Talisman can still be saved with no requirements, but this helps to prevent users unintentionally leaving the requirements blank

Moved "Clear Backgrounds" button from the "Globals" tab to the piece selection panel

Various other UI tweaks - all popup dialogs (settings, hotkeys, about, etc) now display centre-screen, the About dialog and Validate Level feature now have their own dedicated hotkey combos, menu items have been grouped where possible to reduce clutter, etc.
To address Giga's specific suggestions in the OP:
Piece search barWould this be something that could search for a piece by name, regardless of which style it's in? If that's the case, this would be great, but I might need a bit of help with implemention; it's by no means a trivial addition.
Grid based Piece viewerDo you mean draw the alignment grid onto the canvas, or something else?
Force Lemming typeIt's already possible to do this using the "theme" field, simply select whichever style the sprites are associated with (e.g. flopsy_soniclems, gigalem_millas, mantha_ghostbusters, strato_lems_spider-man, willlem_lemminas, etc) - maybe you mean something else?
Pop out Stage viewerNot sure what you mean by this one