To me, the EC client should be stricken of the "Beta" tag when...
1. The modding capability is finished (last I checked, we still can't load XML files without needing a hack).
2. It runs reasonably well on older computers (lower quality graphics, fewer special effects, and a lower frame rate than new computers, of course).
3. Everybody who can use the client for 5 minutes can also use the client for 5 hours without performance degradation.
4. Using something in the 2D client doesn't break that item for EC client players, and vice-versa (like books).
After that is done... I'd say that the client should no longer be considered a beta (but individual features can, and it would be awesome if there was a beta channel and a stable channel for those of us who are adventurous).