Material Design
A bold, colorful, and responsive UI design for consistent, intuitive experiences across all your devices

• Responsive, natural motion, realistic lighting and shadows, and familiar visual elements make it easier to navigate your device
• Vivid new colors, typography, and edge-to-edge imagery help to focus your attention
  

Notifications
New ways to control when and how you receive messages - only get interrupted when you want to be

• View and respond to messages directly from your lock screen. Includes the ability to hide sensitive content for these notifications
• For fewer disruptions, turn on Priority mode via your device’s volume button so only certain people and notifications get through. Or schedule recurring downtime like 10pm to 8am when only Priority notifications can get through
• With Lollipop, incoming phone calls won’t interrupt what you’re watching or playing. You can choose to answer the call or just keep doing what you’re doing
• Control the notifications triggered by your apps; hide sensitive content and prioritize or turn off the app’s notifications entirely
• More intelligent ranking of notifications based on who they’re from and the type of communication. See all your notifications in one place by tapping the top of the screen

Battery
Power for the long haul

• A battery saver feature which extends device use by up to 90 mins
• Estimated time left to fully charge is displayed when your device is plugged in
• Estimated time left on your device before you need to charge again can now be found in battery settings
  

Security
Keep your stuff safe and sound

• New devices come with encryption automatically turned on to help protect data on lost or stolen devices
• SELinux enforcing for all applications means even better protection against vulnerabilities and malware
• Use Android Smart Lock to secure your phone or tablet by pairing it with a trusted device like your wearable or even your car
  

Device Sharing
More flexible sharing with family and friends

• Multiple users for phones. If you forget your phone, you still can call any of your friends (or access any of your messages, photos etc.) by simply logging into another Android phone running Lollipop. Also perfect for families who want to share a phone, but not their stuff
• Guest user for phones and tablets means you can lend your device and not your stuff
• Screen pinning: pin your screen so another user can access just that content without messing with your other stuff
  

New Quick Settings
Get to the most frequently used settings with just two swipes down from the top of the screen

• New handy controls like flashlight, hotspot, screen rotation and cast screen controls
• Easier on/off toggles for Wi-Fi, Bluetooth, and location
• Manually adjust your brightness for certain conditions. Then, adaptive brightness will kick in based on ambient lighting
  

Connectivity
A better internet connection everywhere and more powerful Bluetooth low energy capabilities

• Improved network handoffs resulting in limited interruption in connectivity. For example, continue your video chat or VoIP calls without interruption as you leave the house and switch from your home Wi-Fi back to cellular
• Improved network selection logic so that your device connects only if there is a verified internet connection on Wi-Fi
• Power-efficient scanning for nearby Bluetooth low energy (“BLE”) devices like wearables or beacons
• New BLE peripheral mode
  

Runtime and Performance
A faster, smoother and more powerful computing experience

• ART, an entirely new Android runtime, improves application performance and responsiveness
  
  • Up to 4x performance improvements
  • Smoother UI for complex, visually rich applications
  • Compacting backgrounded apps and services so you can do more at once
    
• Support for 64 bit devices, like the Nexus 9, brings desktop class CPUs to Android
  
  • Support for 64-bit SoCs using ARM, x86, and MIPS-based cores
  • Shipping 64-bit native apps like Chrome, Gmail, Calendar, Google Play Music, and more
  • Pure Java language apps run as 64-bit apps automatically
    

Media
Bolder graphics and improved audio, video, and camera capabilities

• Lower latency audio input ensuring that music and communication applications that have strict delay requirements provide an amazing realtime experience
• Multi-channel audio stream mixing means professional audio applications can now mix up to eight channels including 5.1 and 7.1 channels
• USB Audio support means you can plug USB microphones, speakers, and a myriad of other USB audio devices like amplifiers and mixers into your Android device
• OpenGL ES 3.1 and Android extension pack brings Android to the forefront of mobile graphics putting it on par with desktop and console class performance
• A range of new professional photography features for Android Lollipop that let you
  
  • Capture full resolution frames around 30 fps
  • Support raw formats like YUV and Bayer RAW
  • Control capture settings for the sensor, lens, and flash per individual frame
  • Capture metadata like noise models and optical information
    
• State of the art video technology with support for HEVC main profile to allow for UHD 4K 10-bit video playback, tunneled hardware video decoding to save power and improved HLS support for streaming
  

OK Google
Easy access to information and performing tasks

• Even if your screen is off, you can say "OK Google" on devices with digital signal processing support such as Nexus 6 and Nexus 9
• Talk to Google on the go to get quick answers, send a text, get directions and more
  

Android TV
Support for living room devices

• User interface adapted for the living room
• Less browsing, more watching with personalized recommendations for content like movies and TV shows
• Voice search for Google Play, YouTube and supported apps so you can just say what you want to see
• Console-style Android gaming on your TV with a gamepad
• Cast your favorite entertainment apps to your big screen with Google Cast support for Android TV devices
  

Accessibility
Enhanced low vision and color blind capabilities

• Boost text contrast or invert colors to improve legibility
• Adjust display to improve color differentiation
  

Now in 68+ languages
15 new additions

• Basque, Bengali, Burmese, Chinese (Hong Kong), Galician, Icelandic, Kannada, Kyrgyz, Macedonian, Malayalam, Marathi, Nepali, Sinhala, Tamil, Telugu
  

Device set up
Get up and running in no-time

• Tap & go: instant set up of your new Android phone or tablet by simply tapping it to your old one (requires NFC)
• Whenever you get a new Android phone or tablet, you can bring over your apps from Google Play automatically from any of your old Android devices
  

And a whole lot more

• Tap & pay: easily manage multiple payment apps by quickly switching between them
• Print preview and page range support
• Revamped display for battery, Bluetooth, data usage, and Wi-Fi settings and new search functionality
• New device level feedback for Nexus devices in Settings > about phone > send feedback
• Easier sharing with
  
  • Improved ranking of your options within the share menu
  • Android Beam: lets you share a file with someone nearby by gently tapping the two devices together
    
• Where supported by the hardware, your device will wake up as soon as you pick it up or tap the screen twice
• Improved hardware keyboard accessory support including support for multilingual, emoji input, search key, and improved app and system key chords

Ich hatte mit meinem s5 nur Probleme mit lollipop (Akku hat nicht mal halb so lang gehalten), hab jetzt per odin wieder kitkat drauf gespielt und alles läuft. Ich hab gelesen das es vielleicht daran lag, das ich lollipop nicht sauber drauf gespielt habe... ich hab es einfach per Kies gemacht. Was War daran unsauber und wie macht man es sauber? Jeder schreibt in anderen Foren das die Leute selbst schuld sind wenn sie es unsauber drauf spielen aber keiner sagt wie es sauber geht....

Das ist auf projekt ara eh drauf ^^

Jein. Ara wird Lollipop haben, aber in einer modifizierten Variante. Das Ara-Android wird so umgebaut dass es mit Hardwareänderungen ohne Neustart problemlos umgehen kann, immerhin lässt sich alles bis auf Display und Prozessor austauschen ohne das Ding abzuschalten.
Ich denke, wenn sich das Projekt gut verkauft wird Google früher oder später beide Fassungen wieder vereinen.

Ich werde mir ein ara holen. Bin erst vor einer woche auf mein (erstes) smartphone drauf getreten und jetzt hab ich einen Riss über den gesammten bildschirm. Hoffentlich überlebt das noch lange genug ._.

Das wäre dann aber nicht mehr sauber...

Eine saubere Neuinstallation beinhaltet das löschen von sämtlichen Partitionen (system, cache etc.) und eine darauffolgende Neuinstallation mittels eine ZIP Datei über CWM.
Alles andere ist nicht sauber.
Und keine Backups wiederherstellen.

Und wie funktioniert das ganze? Ich hab von Smartphones nicht so viel Ahnung

Erst einmal muss das Gerät gerootet werden(und der Bootloader entsperrt).
Dann kann man eine CWM(Clockworkmod Recovery) installieren, also ein Wiederherstellungsmenü das mehr Optionen bietet als das originale.
Und mit dieser CWM kann man nicht nur das Gerät in den Werkszustand zurückversetzen sondern auch verschiedene Custom ROMs(modifizierte Android-Versionen) mehr oder weniger per Knopfdruck flashen.

Dazu gibt es einige ausführliche Tutorials, denen man eigentlich nur Schritt für Schritt (genau) folgen muss.