The new cell insertion point icon can be suppressed by using the option
in a notebook, style sheet or a CreateDocument statement.
I hadn't noticed the "Enable autocompletion..." option in Interface preferences. Thanks for bringing it to my attention.
But instead of a popup delay, or as an alternative, I would much prefer a choice to enable autocompletion by using Ctrl+K. Then I could get it as fast as I want, or not at all.
I would still like to see the notebook interface optimized for the experienced user and not for na=EFve beginners. That means as clean as possible and to me that means a blank sheet of paper. Beginners can learn from tutorials, or perhaps there could be a "training wheels" option in Preferences (normally off).
Nevertheless, there is a problem with Mathematica even for experienced users. That is getting information on the existence and nature of the large number of routines available, and selecting those that might be useful at some point in your work. I think this is probably part of the objective in the Suggestion Bar and Predictive Interface. But I think they aimed it in the wrong direction. Instead of writing code for you, they should be directing you to information that will help you better write your own code. I don't know what is behind the "predictive interface", maybe cellular automaton agents?, but perhaps a user could select a cell and on the context menu bring up a temporary menu with direct links to relevant Guide pages (and Tutorials). This would be a shortcut rather than having to drill down (and around) from the DC Home page. Being able to go directly to relevant Guide or Tutorial pages based on context would be a great help.
It should also be noted that Function and Guide pages have a toolbar at the top that gives the various paths to the page. Starting from a lower level you can work back up the structure by clicking on the links.
It would also be really nice to have a tabbed Documentation Center as an adjunct to multiple windows.
By default, the new interface doodads -- autocompletion, Suggestions Bar, and dynamic highlighting -- are enabled. And this is probably a good thing for the new Mathematica user who just wants to start using the system as quickly as possible without a long learning period.
The experienced user need only open Preferences/Options and on the Interface tab uncheck the corresponding items.
In the case of autocompletion, there's even the option to leave it enabled but set a popup delay.
For the new cell insertion point, the little + icon is hardly distracting. In fact, to the unknowing eye, it merely suggests this is a place where you can add some input. It's not until you click the + sign that you see the choices, including free-form/WolframAlpha input.
And the + icons gives those who want it another way to insert a Text cell without having to revert to the main menu or the keyboard shortcut equivalent.
Perhaps there's a way to use the Option Inspector to turn off, too, the + icon at the new cell insertion point; I haven't discovered it yet.
I agree that it would be useful to have the choice of these interface doodads readily available to unset or reset on a menu or a palette.