Improving and expanding hashtags usability.
Current functionality of hashtags in Diaspora has several limitations. I'll list them first (if you can come up with more - feel free to comment):
User can only follow a straight list of tags (i.e. following is defined with logical OR).
User can't filter tags for not following (i.e. not to see some content which has them).
Viewing options of followed tags in the UI are limited to either viewing all followed tags, or only one.
Search is limited to one tag.
Hashtags themselves don't allow whitespace in them which is caused by the syntax restrictions and results in awkward unreadable notations like "#areallylonghashtagyoucanbreakyoureyeson", or people trying to come up with workarounds (like using underscore or camel notation, which proliferates incompatible tags and defeats the purpose of searching by them).
To address these issues, several improvements can be made (these proposals match the problems described above).
1-2. Following by default can still be defined as a simple list (which is equal to tag1 OR tag2 OR tag3 OR ... OR tagN). Interested user can be given an advanced option to define a more complex boolean expression for following hashtags. I.e. allow using AND, OR, NOT and parentheses. This will cover both 1 and 2, allowing way more flexible method of following and filtering data.
When using the UI for viewing, one should be given a way to view one, several or all (multiple select) of those hashtags. This is sufficient for the UI case. More complex view will be covered with search (4).
Search should allow boolean expressions, the same way the following in the proposal (1-2).
Syntax of hashtags can be expanded. For example it can allow such form in addition to simple not whitespaced tags:
#(some phrase with spaces)
In the final text it can look like a hyperlink without parentheses:
#some phrase with spaces
Parentheses are used just for definition, to delimit the beginning and the end of the tag. This will give a clean way to avoid multiple incompatible notations for complex multiword hashtags.