Google always try to make all of their products faster and easier to use, so that's why Gmail now offers a completely new compose and reply features.
The first major change comes in the form of your compose box sitting on top and to the right of your inbox: this makes referring to other emails far easier. Google have also improved the automatic saving of your email drafts so if you accidentally close the compose window Gmail saves the draft automatically, so you don't lose anything. The new compose pop-up also allows you to compose several mails at once for those times when multitasking is the only option!
The new compose has slimmed down some of the less important features to dedicate more space to your composition: the CC and BCC fields are hidden away and have a compact link to make them appear; the email address autocomplete now shows the profile picture of the recipient that makes finding the right contact easier and quicker. The formatting tools now pop-out of a button and are categorised to make them compact. If you want to find where the indent tool have disappeared to in the new Gmail compose window you will now find it on the Alignment button. you will find it much easier adding inline images and attachments also.
Another new feature you will find useful is Insert files using Drive: this uses Google Drive to upload files to and share with the recipient, which means you can "send" a huge file via email instead of only 25MB! You can make the files inserted through Drive secure by only sharing with the recipients and then they will need a Google Account, but if your recipients don't want to get a Google Account you can share to Anyone with the link.
Why stop with files? Now you can send whole folders too! Just make a folder in Google Drive and upload files you want to share. Once uploaded you can use the Insert files using Drive icon to select the folder and share with whoever received the mail. This can save time and effort as consolidating the files into a folder means you only need to share one item and you can easily manage those you can access the folder from Google Drive, just incase you sent it to the wrong person!