While it's true that Apple is phasing out iPhoto in favor of its new Photos app, iPhoto is still an incredibly powerful program and includes many features you likely know nothing about. If you don't plan on moving to Yosemite 10.10.3, or if you elect to keep using iPhoto when you do--yep, it'll still work!--here are four useful things that iPhoto can do that even the new Photos app currently cannot.
Add captions to slideshows
If you don't relish the idea of narrating your slideshows, try adding captions instead. Select a photo in an album, or in Photos or Events view, and then click the Info button in iPhoto's toolbar. In the panel that appears, enter a title and/or a description for your photo (titles appear in a slightly larger point size than descriptions).
Next, select the album or photos you want in your show and choose File > New Slideshow (this creates a saved slideshow in the Projects section of the source list). Click the Themes button in iPhoto's toolbar and choose Ken Burns or Classic, and then click Choose. Click the Settings button in iPhoto's toolbar and click the All Slides tab.
From the Show Captions menu, pick the kind of caption you want--both titles and descriptions are turned on in this example. You can also add captions to instant slideshows--those generated by clicking the Play button in iPhoto's toolbar--by wiggling your mouse to summon the floating menu and then clicking the gear icon to open slideshow settings. (As of this writing, Photos' slideshow settings are limited and don't contain a Show Captions menu.)
Adjust Levels using a live histogram
A popular way to correct color in advanced image editors such as Photoshop, Photoshop Elements, Lightroom, and Aperture is to perform a Levels adjustment. You can do the same thing in iPhoto using the Adjust panel's histogram.
Just select a photo and click the Edit button in iPhoto's toolbar, and then click the Adjust tab at upper right. The histogram that appears is a self-updating collection of tiny bar graphs representing the dark and light tones in your photo; darker shades appear at left and lighter ones at right. The taller the bar, the more pixels you have at that level of brightness. (Another way to think of a histogram is to imagine your photo is a mosaic, and that the individual tiles have been separated into same-color stacks. The taller the stack, the more tiles you have of that particular color.)
If the bar graphs in your histogram cover all the territory from left to right, you already have a roughly even distribution of dark and light tones in your picture, so you're probably in good shape. But if the graph comes up short on either the left (dark) or right (light) side, you need to make an adjustment to spread out the photo's information.
To do it, drag the left and right handles on the Levels slider inward to where the bar graphs begin. Doing so resets your black (shadows) and white (highlights) points. Next, adjust your photo's midtones by dragging the middle handle slightly left or right until the photo looks good to you. (While you can choose to view a histogram in the Photos app's Adjust panel using the Add button, it doesn't have Levels sliders.)
Recover details in the highlights
Photos captured in raw format contain more information than their JPEG brethren, so iPhoto lets you do a little more with them. For example, there's a hidden slider in the Adjust panel that you can summon to recover lost detail in the highlights.
To use it, select a raw format photo and click the Edit button in iPhoto's toolbar, then click the Adjust tab at upper-right. Press and hold the Option key on your keyboard and the Exposure slider becomes Recovery instead; just drag the slider to the right to bring back any detail that was lost in the highlights. Release the Option key and the slider goes back to adjusting Exposure. (As of this writing, the Photos app doesn't include a Recovery slider.)
Find every photo without a keyword
Keywords are descriptive words--like food, sunsets, or Fido--that you can use to label and categorize your photos, regardless of which album or Event they're in. The beauty of iPhoto keywords is that they're searchable, which lets you quickly find specific photos.
To find all the photos that don't have keywords, choose File > New Smart Album and name it "No keywords." In the resulting dialog box, set the menus to "Keyword," "is," and "None," and then click OK. iPhoto scours your entire library for photos that don't have any keywords applied and plops them into your new smart album. The next time you've got a hankerin' to go keywordin', those photos will be waiting for you. (As of this writing, the Photos app's smart album variables are limited, and don't include the "None" value.)
As you can see, iPhoto is full of surprises, and there may yet be reason enough to keep using it even after the Photos app hits the scene. Until next time, may the creative force be with you all!