Drupal 7 got "CCK" integrated with key concepts like entities and fields. Entities encapsulate the description of a content item while fields are the individual editable pieces of entities which have their widgets, validators, storage and rendering well defined. That entities were introduced for nodes, taxonomy and users, it became possible to attach fields to these, and unify their configuration, storage and rendering. This coupled with translatable fields and the contributed Entity translation and Title modules enabled these objects to be translated the same way, reusing concepts, workflows and implementation.
As part of the multilingual initiative, we are trying to (a) unify and (b) broaden support for translatable objects. Fieldable entities are the ideal backend in terms of translatability, especially when all their translatable values actually use fields. Therefore part of the Drupal 8 goals for multilingual support is to get the Title module or similar functionality to core (issue at http://drupal.org/node/1188394) as well the field translation UI based on / from Entity translation (issue at http://drupal.org/node/1188388).
That all helps with existing entities. However, we encounter structured objects that for one reason or another don't use entities. Think blocks, menu items, contact forms, views, and so on. While fields and entities are great for translatability, the custom implementation of non-entities is the other end. We discussed in a previous post that structuring non-entity objects with fields or field like metainformation, widgets, permissions and rendering with associations to the original objects is needed for clean multilingual implementations. However, there are probably more objects that would make sense to convert to entities, which in itself will not only make them more powerful, customizable and standard to work with, it will also help tremendously with translatability.
Blocks
There are so many efforts to make entities out of blocks, it is hard to count them even :) Dave Reid has a project at http://drupal.org/project/block, fabsor has a sandbox at http://drupal.org/sandbox/fabsor/1158058, there is yet another project at http://drupal.org/project/bean and another at http://drupal.org/project/block_api. The core issue is at http://drupal.org/node/430886
Menus and menu links
Again, Dave Reid has multiple issues about this, the current one is at http://drupal.org/node/916388
What else?
Would the above two make sense to make entities? What other Drupal objects would be good entities? Fieldable contact forms anyone? Aggregator sources that you can assign an optional image to? Also, let's not confine ourselves to a wishlist. Who is going to help get these done?
