- the infobar in the effects properties is too prominent imo, I would remove it.

The previous code that used text directly in the listview seemed better to me as it didn't use additional space; however, I guess gtk infobar was used for standardization/maintainability purposes...? Not convinced it's the best everywhere?

Among the first things I'd like to see fixed is the handling of clip selections for drag and dropping in the timeline and in the effect clip list. I would recommend the following changes:
1- If dragging an effect to the timeline, the clip on which it is dropped must get selected automatically before applying the effect
2- If multiple clips are selected, the clip effect properties UI is disabled with a message saying that you can't add effects to multiple clips at once (or something along those lines). Note: there will be a better way, someday, to apply the same effect to multiple clips.
The rest, so far, does not need to be changed.

#1 is quite important to me because I think there are a couple of bugs that depend on this (I've seen pitivi get "confused" easily about which effects to display in the clip effects properties list).

Also, remember the bug where pitivi "seeks" forward a few frames when *adding an effect* a clip? It's still there, but the playhead comes back to the correct frame when you toggle the effect on/off or edit its settings. So it happens only when you "add" the effect. Strange.

- atm effects are the only thing inside clip properties and yet they are
  contained in a gtk expander. I'd remove the expander since it's not really
  useful right now and it looks ugly imo.

I guess I agree with this as a temporary measure (until we can edit clip properties such or have a clip speed editor, for example)...

- the effect properties pane moves too much, depending on the properties a
  specific effect has. I'd make it request a sensible default size and put the
  content in a scrollable.

Yes. Make that scrollable and nail its friggin' position down (save it automatically). Oh, and make sure it has a sensible default size too (initially, we can barely even see a single effect in the list!).