Loss in interest on refunds
In case you claim a refund in your return of any advance tax paid/TDS, you would lose some of the interest (currently 6% per annum paid by the tax department) on such refund.
The interest on refund is normally computed from April 1 of the assessment year (the year immediately following the financial year for which the return is filed) till the date of grant of refund.
However, in case of a belated return (i.e. return filed after due date) interest is computed from the actual date of filing the return till the date when refund is granted.
This means loss of the interest that would have been paid for the period April 1 till date of filing the return. Even if you file the return one day after the due date you would be losing interest for at least four months - April, May, June and July (presuming due date is not extended beyond July 31).
No carry forward of losses
If you file a belated return you cannot carry forward losses (except loss from house property).
Losses under the following heads of income: Income from business and profession including speculation business, capital gains, and income from other sources cannot be carried forward in case a belated return is filed by the tax payer.
The return filer will not be allowed to carry forward these losses even if all taxes have been paid in time if the return is belated.
Delayed return where tax remains unpaid
If you have any unpaid tax liability, filing your return after the due date would result in levy of penal interest @ 1% per month from the due date of filing the return till the actual date of filing.
This would be a heavy and avoidable payout.
Penalty for late filing
If you do not file your tax return by due date, there can be penalty as high as Rs.10,000 depending upon the amount of your income. This is definitely a avoidable charge. In case you have additional information post filing of your return, you can always revise your ITR later, but before 31st March.