As a Strategy consultant, it's just not seen as top-tier (BCG, Bain, McKinsey are still very much seen as the top 3). While this isn't necessarily something that prevents Deloitte "alums" from doing anything, you get the strong sense that the alumni network, post-consulting job opportunities, and entrees into business school and management on the "industry" side are just not as good as those other firms.
However, I think it's a great place to START a career. Joining as an analyst was great for me, and has propelled me to later career success. However, if I were to re-join the firm after my MBA, I would see it severly undermining my career opportunities going forward. Perhaps that's why all my colleagues at my Ivy League business school in New England talk about going to the big three - but pretty much no one wants to go to Deloitte. Which I think is the correct, and rational, judgment. So if you join, join early and then move on. The later you join in your career, the more limited your opportunity set and the value of going to the firm in the first place.
As a side note, that's why I believe that the talent at the analyst level is comprable to, if not better than, the big 3. However, the higher you move in the firm, the more it pales in comparison to our competitors.