For the removable battery, my best guess would be that it was eliminated due to the new design of the phone and the newer battery technologies. There has recently been a new type of battery technology created that splits the battery up into many smaller, thinner cells to allow a much thinner overall thickness. This also allows any unused space inside the device's case to be used for additional cells, which increases the overall battery capacity. Apple is using this type of design in the new Macbook Pro that was recently announced. Read more about it at https://www.apple.com/macbook/design/. The fourth picture from the bottom illustrates this new battery concept. Samsung may have a similar design, which would obviously prevent the batteries from being removable, but theoretically would reduce the need for a removable battery.
As for the waterproof aspect, this may be a strategic move to allow the waterproof feature to be used as a value added feature on a future model or variant of the Galaxy S6, or may simply have been cost prohibitive with the new hardware and/or design. It also could have been that adding the components necessary for the phone to be waterproof would have taken away from the aesthetic design. They clearly put a lot more focus on the design than they have in the past, and design unfortunately sometimes comes at the sacrifice of function.
That is why people may stick with S5... Different strokes for different folks.. With ability to use ANY wireless charging pad and fast charging (53% charge in 30 minutes,, 4hours battery life in 10 minutes) and ultra power saving mode (1 day of batteru life at 10% power) there may not be as much a need for replaceable battery.. Plus they woukldd compromise on premium features..such as metal case. The antennas are now part of the back case to make it thinner and lighter as well.
For waterproofing looke at Lifeproof cases.
The Samsung people at BestBuy didn't have a answer as to why the waterproof feature was not going to be on the s6. It could be many things. A lot of people with the s5 complained that it would break off and could not be fixed. I checked into that and was told (by the Samsung center people) that you can order a replacement. I have only had my s5 for 5 months and am careful when I open or close the latch. It could be because the s6 is a little thinner, and they would have to redesign the latch. Maybe they thought more phones would be ruined from water & get more sales. As far as the s6 being a sealed device they are putting a expiration date on all of the s6s. Some people like myself may keep phones more than 2 or 3 yrs. Making the s6 sealed, guarantees them more future sales. What jrwaz said about the new batteries is true, but they can still only be charged so many times. They also get weaker and don't hold a charge as long. I am sure whatever reason Samsung is going to this closed device is to their benefit. I feel they may lose customers that purchased Samsung over iPhone because they could add a storage card & new battery. If anyone knows why they are doing it, please comment.
a large majority of people don't care for the MicroSD card slot. Often they don't even know about the slot or even bother putting something into it. For example the people at the office I work for. There are 3 people I know out of 300 employees at the office that use their microsd card slot, or even have one.
removable battery is subjective. A lot of people can live without it and depending on your usage dictates battery life. Heavy users may forego them, but a vast majority will either upgrade or probably have their devices last 3-4 years anyway. I have devices from 4 years ago and the battery still last me at least 14 hours. I can sport my Fascinate if you want as an example
Personally I like the card slot, but the removable battery is like meh... Phones generally last me at least 14 hours before a charge and some last me 1-2 days. My Moto X for example I have made 38 hours more than a handful of times.
It all comes down to design. In order for them to achieve a super slim profile they needed to cut back on features such as the SD card slot which takes up valuable (internal) space. And with a unibody construction a removable battery just isn't feasible. Neither of which are a deal breaker to me, I plan on purchasing a portable battery pack and using cloud storage for the vast majority of my photos and music.