There are some things that you should always be careful to look out for:

Inbuilt wardrobes:

Many properties have inbuilt wardrobes, this means that you will get used to seeing bedrooms with minimal furniture; typically just beds and side tables. After looking at lots of similar rooms, I have often seen people fail to notice when there are no inbuilt wardrobes in a bedroom that doesn’t have room for a free standing wardrobe. Be careful not to make that mistake as many Spanish properties are low on storage space as it is.

Storage Space:

On that note, keep storage space in mind, especially if you are going to be letting friends and family use your holiday home and want some private space to keep your own things in. Not having to pack everytime you go away is a huge luxury (and saves a lot of money on airlines that charge for luggage). 

Parking:

A car is a necessity in many areas of Spain and a parking space is often a necessity. Many apartment blocks will have underground parking that is sold separately to the apartment (they two will have separate deeds). Make sure that you have parking if you need it and that, if the space is separate to the property, it is included in the sale.

Utilities:

Make sure you know where your utilities are coming from. Even in some relatively urban areas you may find that the property is connected to a septic tank rather than the sewer system, and water sometimes comes from deposits rather than the mains. In many areas gas is supplied in bottles however in some resorts it is supplied by the resort. 

Sun and Shade.

When I first started working with off plan properties you could often tell which way a site plan should be orientated if you know the nationalities of the buyers of the sold units. Spaniards would buy North facing properties to escape the heat, Northern European buyers would buy the south facing units. It's worth noting that it’s not just a cultural difference, the Spanish use their holiday homes in the peak of summer and a lot of Northern Europeans would seek the winter sun.In an ideal world you want to find a place that has both sun and shade available throughout the day all year round, however, if you opt for south facing only look for Toldos (sun shades) and check their condition. Trust me, come summer you will want them and if they're not there already then they can be expensive to fit. 

Air-conditioning.

Most properties these days will have air conditioning installed already. Take a good look at the units and take down the model name and number, if possible check that they work (believe it or not I have seen properties when interior aircon units looked fine, but the outdoor units that feed them had been stolen). Air-con units all look pretty similar but there are a huge range of manufactures and models with varying power outputs and efficiency ratings. It's worth looking up the units and getting an idea of age and running costs as, if they are old and efficient they may need replacing soon and this can be factored into your negotiations. I have mentioned before that no matter what the condition of the Aircon you should have it cleaned and serviced when you move it to eliminate the risk of legionnaires disease having contaminated the system. 

Appliances.

As with the Aircon, take a look at the appliances, make sure that they work and take a note of the models. If they are new and efficient then they are a selling point. If they are old and tired, not only will you have to replace them but you might be lumbered with the task of removing and disposing of them. 

Bins.

This is something I have referred to before but will mention again. In most areas, refuse is collected from communal wheelie bins rather than door to door. This means that it can be collected much more regularly which is essential in the summer heat. Keep an eye out for where your communal bins are as in the summer months when they are collected in the middle of the night (and can smell quite bad during the day) you don’t want them to be outside your bedroom window.