TX Interrupt, Enhanced Privacy, and Capplus are probably the most notable non-standard features that require a license.
The SL series is surprisingly rugged from a drop/impact standpoint, however the internal speaker audio output is very weak and pathetic. Its best used with the operations critical Bluetooth accessories or wired earpiece.
Note that the USB charging is pretty particular, so although it is a common Micro USB, not every charger will work. I believe the OEM chargers are 5V 850ma.
I have seen a fair amount of button failures however (volume up/down, ptt, etc)
SL 7550e is now three watts, and the battery life is reasonable to very good with the extended battery. Don't even bother with the slim.
With a properly planned and high quality system install (proper pattern antennas for the building or area (not some junky high gain fiberglass on the roof of an office tower), good feed line like heliax, combiner/multicoupler instead of a bunch of duplexers, SLR series repeaters, everything done A+), the SL7550e is an extremely viable option, and can be your "standard issue" without a doubt. If there are areas where the XPR will work and SL won't, and the usage of the radios is even slightly important, it means you need another site, BDA, DAS or your system install isn't what it should or could be.
For some TRBO systems, the digital voting feature can really help even out the talk in range on the SL.