Most keyboard instruments of the baroque Era as well as early pianoforte had fewer keys than the current 88-standard (some modern piano like the Bosendorfer Imperial have 97 keys, 9 additional keys in the bass), and much of Bach keyboard works for instance can be played on such a restricted keyboard.
With restricted high notes you will have difficulties with many of Chopin's and Liszt's works where the composers take hold of the whole instrument and use replication of elements among successive octaves for effect.
Many pieces of composers such as Scriabin and Debussy also use the whole range of the piano.
Another thing you would miss is playing four-hands, but I guess this is not the kind of activity you are into.
If your new keyboard is able to transpose 1 octave up or down, you may be able to play more pieces by adjusting the range to the work you want to play, as if you were shifting your chair left or right in front of your piano.
music.stackexchange.com/users/48/ogerard
I have a 61 key and don't find it a problem, but then other people used to larger keyboards do.
Just make sure it has handy octave-transpose buttons and you can do a bit of quick transposing mid-song if you need it.
music.stackexchange.com/users/298/Tim
If you really like to play flashy virtuosic pieces, then it might be a problem.
But the highest octave is hardly ever used, and if it extends to an A on the lower end, that should cover most pieces in the standard repertoire.
Looking at a Google image search of 61 note keyboards, it seems that many of them end on a low C (C2).
It would be a safer bet if it extended at least down to the A, but those lower notes are often used for octave doublings, in which case you'd probably be able to get away with it.
music.stackexchange.com/users/85/cotroxell
Yes, it will restrict your repertoire.
I have a 61 key synth, and there are occasions when I have to adapt a piece 'to fit'.
Not very many songs, mind.
It doesn't make me regret buying it 20 years ago.
There is a dedicated transpose key that I can configure from -24 to +24, so the theoretical range is greater than 88 keys, however, despite it being very easy to use, it can still be impossible/impractical to execute whilst playing (you also have to shift your hands pretty quickly to compensate for the left or right hand that didn't need to transpose).
I only usually transpose keys between songs.
You can get an 88 key midi controller very reasonably: http://www.amazon.co.uk/M-Audio-9900-40832-00-Keystation-88es/dp/B0006676A0/ref=sr_1_3/280-5526809-4799532?s=musical-instruments&ie=UTF8&qid=1400103634&sr=1-3&keywords=88+key+keyboard+weighted
So you can enjoy the best of both worlds, space and pocket permitting.
Since the 61-key market does have some fine workhorses.
Many 88-key models are also available as 61-key models and can be half the price.
So you can save some money if you already own an 88-key controller, and a compatible cable, of course.
music.stackexchange.com/users/9877/Lee Kowalkowski
I went from a board with 61 keys to one with 73.
It was a vast improvement.
I still miss the very lowest keys, not so much the highest octave.
Happily I have my old upright at home for when nothing will do but that low A, or Bb.
When gigging, I need to stay away from too much bass anyway, so it is not so bad.
But if this is your only keyboard, I expect you will feel very cramped on a 61.
A 73 would be better.
But if you can find (and afford) an 88 - with an acceptable key feel and that is not too back breaking - pick it up.
You are a pianist as opposed to a keyboard player.
I think there is a distinction (both are wonderful assets to humanity, of course)
so weigh any advice you get based on the speaker's orientation to his or her instrument.
music.stackexchange.com/users/15370/memphisslim
You might experience issues.
I'm learning Liszt's Hungarian rhapsody No. 9 right now and for some of the highest notes, an ossia for 66-key pianos is given.
Not sure if they'd fit in to your 61-key one, though.
music.stackexchange.com/users/2725/H2CO3
Whatever you do, don't get rid of the 88 key digital piano and switch down to a 61 key digital keyboard!
The only reason for doing this is if you are into organ music, thus not needing that additional octave at the top and additional octave and one third at the bottom of the keyboard!
Like others say, hitting (or even finding) that octave transposition feature in mid selection is a drag and will distract you from the music enough to make a mistake (or variation as us musicians like to say to minimize errors in performances)!
Yes, it will limit you in your playing of classical pieces of music and you'll definitely want to hold on to this one!
If you do indeed want to go with 61 keys, try to get at least two, if not three or more so that you can have more musical range for organ pieces!
What I would do is go with a digital computer organ or even have a pipe organ built if it was in your budget as well as meeting your portability needs!
music.stackexchange.com/users/10255/Zach LaFleur
Please remember that when using a MIDI keybaord of 61 keys,, it is extremely easy to transpose the key, for instance -12 semitones or an octave.
giving your 12 more bass notes at the expense of the highest notes...
This will help a bit for certain pieces.
The transpose function also allows you to cheat, by playing a piece in any key that suits you and not learning new notes.
music.stackexchange.com/users/10699/sonofhendrix
If you already play classical music surely you can answer this yourself to some extent.
How many times do you play keys in the top and bottom octave in the music you've played?
Classical music covers a massive range, I think if you were utilising those extremes of pitch you'd probably remember!
music.stackexchange.com/users/15313/Mr. Boy
I'm planning on trading my 88-key digital piano for a 61-key portable music workstation as I'm more inclined into rock keyboards as of late than classical music. However, I do enjoy playing classical parts, and I wonder if buying a keyboard with a restricted range will restrict my repertoire much. A 61-key keyboard essentially removes the first and last octaves from the 88-key traditional piano.  Are there many pieces that would be deemed unplayable without these octaves?
Will a 61-key keyboard severely restrict my piano repertoire?
music.stackexchange.com
<piano><repertoire><keyboard>
music.stackexchange.com/questions/492
music.stackexchange.com/users/291/Diego
S0: Most keyboard instruments of the baroque Era as well as early pianoforte had fewer keys than the current 88-standard (some modern piano like the Bosendorfer Imperial have 97 keys, 9 additional keys in the bass), and much of Bach keyboard works for instance can be played on such a restricted keyboard.  With restricted high notes you will have difficulties with many of Chopin's and Liszt's works where the composers take hold of the whole instrument and use replication of elements among successive octaves for effect. Many pieces of composers such as Scriabin and Debussy also use the whole range of the piano.  If you really like to play flashy virtuosic pieces, then it might be a problem.   Some classical pieces will be able to be played, eg Bach. However there are some classical pieces that you will not be able to play, Chopin, Liszt, Scriabin, Debussy and will restrict your repertoire. 
Some classical pieces will be able to be played, eg Bach. There will be some classical pieces that you will not be able to play, Chopin, Liszt, Scriabin, Debussy and will restrict your repertoire.