Plus is great technology but we’ll have to see how it gets adopted. If it can create enough momentum until it gets baked into the next Android phone OS and receives 500.000 users per day then it’s won but until then, there’s work to do.
Personally I think the success purely rests on how it performs in the mobile space.

Things I would like to see on the mobile client:

Collapsed view on Stream. Two posts per page isn’t enough for scanning. I’m using an Android 2.x tablet. it must be even worse on small android screens.

Timeline view. Turn off re-ordering so you can catch-up more easily. I believe Google will change the way popular posts rise to the surface but that’s not the same as a timeline view.

Circles muting. I can creative circles that allow me to chat with, say, people in Germany or people that like talking about food but I can’t listen to those topics specifically yet. I want to mute circles sometimes or just view one circles at other times. It would reduce noise for certain circumstances. An alternative is a layer of topic keywords and circles.
Note, you can watch the stream from a specific circle but unless you manually creative and manage super-circles, its not possible to combine circles. Left and right swipe on main stream in mobile client could easily achieve this.

I want to import my contact groups in Gmail as circles.

A Picasa client would be nice for tagging, organising and even basic editing of photos and videos.

Plus needs a version for big screens.

I would like to be able to mute people but not block them. Blocking in Plus means they can’t hear you!

I would like to tag myself with keywords and I would like Google to present me the tags from others as circles. E.g. I want to see auto-circles based on my contacts tags. Gives me an better idea of what interests people.

On tagging posts. I realise that it could make things complex but have separate circles of keywords, e.g. photography, could be useful. if you tag a post with ‘photography’ it could auto-create a circle of friends that have also used that keyword.

I want auto-uploaded videos to be available in YouTube if I wish.

There are some good opportunities for better widgets.

I want an rss feed of my plus posts.

The api needs to come soon. Devs are chomping at the bit!

I want my to see posts that I make as ‘+1’,  appear in reader.

I want to be able to force-email non-plus users. Maybe there’s something I need to say to my family that’s easier to do through a circle on Plus rather than creating a mail-to list. None of my family are likely to get into Plus very quickly so email remains the lowest common denominator. Of course, this would massively drive adoption like it did introducing people from the desktop client in the first few days of Google Plus.

I would like this post to auto-post to Plus!

Now, where should we discuss this. Here, Plus or Twitter, where it will appear automatically.