The OpenType specification defines the following "feature" tags.
aalt
Access All Alternatesabvf
Above-base Formsabvm
Above-base Mark Positioningabvs
Above-base Substitutionsafrc
Alternative Fractionsakhn
Akhandsblwf
Below-base Formsblwm
Below-base Mark Positioningblws
Below-base Substitutionscalt
Contextual Alternatescase
Case-Sensitive Formsccmp
Glyph Composition / Decompositioncfar
Conjunct Form After Rocjct
Conjunct Formsclig
Contextual Ligaturescpct
Centered CJK Punctuationcpsp
Capital Spacingcswh
Contextual Swashcurs
Cursive Positioningcv01-cv99
Character Variantsc2pc
Petite Capitals From Capitalsc2sc
Small Capitals From Capitalsdist
Distancesdlig
Discretionary Ligaturesdtls
Dotless formsdnom
Denominatorsexpt
Expert Formsfalt
Final Glyph on Line Alternatesfin2
Terminal Forms #2fin3
Terminal Forms #3fina
Terminal Formsflac
Flattened Ascent Formsfrac
Fractionsfwid
Full Widthshalf
Half Formshaln
Halant Formshalt
Alternate Half Widthshist
Historical Formshkna
Horizontal Kana Alternateshlig
Historical Ligatureshngl
Hangulhojo
Hojo Kanji Forms (JIS X 0212-1990 Kanji Forms)hwid
Half Widthsinit
Initial Formsisol
Isolated Formsital
Italicsjalt
Justification Alternatesjp78
JIS78 Formsjp83
JIS83 Formsjp90
JIS90 Formsjp04
JIS2004 Formskern
Kerninglfbd
Left Boundsliga
Standard Ligaturesljmo
Leading Jamo Formslnum
Lining Figureslocl
Localized Formsltra
Left-to-right alternatesltrm
Left-to-right mirrored formsmark
Mark Positioningmed2
Medial Forms #2medi
Medial Formsmgrk
Mathematical Greekmkmk
Mark to Mark Positioningmset
Mark Positioning via Substitutionnalt
Alternate Annotation Formsnlck
NLC Kanji Formsnukt
Nukta Formsnumr
Numeratorsonum
Oldstyle Figuresopbd
Optical Boundsordn
Ordinalsornm
Ornamentspalt
Proportional Alternate Widthspcap
Petite Capitalspkna
Proportional Kanapnum
Proportional Figurespref
Pre-Base Formspres
Pre-base Substitutionspstf
Post-base Formspsts
Post-base Substitutionspwid
Proportional Widthsqwid
Quarter Widthsrand
Randomizerclt
Required Contextual Alternatesrkrf
Rakar Formsrlig
Required Ligaturesrphf
Reph Formsrtbd
Right Boundsrtla
Right-to-left alternatesrtlm
Right-to-left mirrored formsruby
Ruby Notation Formssalt
Stylistic Alternatessinf
Scientific Inferiorssize
Optical sizesmcp
Small Capitalssmpl
Simplified Formsss01
Stylistic Set 1ss02
Stylistic Set 2ss03
Stylistic Set 3ss04
Stylistic Set 4ss05
Stylistic Set 5ss06
Stylistic Set 6ss07
Stylistic Set 7ss08
Stylistic Set 8ss09
Stylistic Set 9ss10
Stylistic Set 10ss11
Stylistic Set 11ss12
Stylistic Set 12ss13
Stylistic Set 13ss14
Stylistic Set 14ss15
Stylistic Set 15ss16
Stylistic Set 16ss17
Stylistic Set 17ss18
Stylistic Set 18ss19
Stylistic Set 19ss20
Stylistic Set 20ssty
Math Script Style Alternatesstch
Stretching Glyph Decompositionsubs
Subscriptsups
Superscriptswsh
Swashtitl
Titlingtjmo
Trailing Jamo Formstnam
Traditional Name Formstnum
Tabular Figurestrad
Traditional Formstwid
Third Widthsunic
Unicasevalt
Alternate Vertical Metricsvatu
Vattu Variantsvert
Vertical Writingvhal
Alternate Vertical Half Metricsvjmo
Vowel Jamo Formsvkna
Vertical Kana Alternatesvkrn
Vertical Kerningvpal
Proportional Alternate Vertical Metricsvrt2
Vertical Alternates and Rotationzero
Slashed Zero
In addition, there are (currently) nine deprecated feature tags which might still be encountered in the wild from time to time:
altv
Alternate Vertical Metrics, deprecatedcrcy
Currency, deprecateddflt
Default Processing, deprecatedjajp
Japanese Forms, deprecatedjp03
JIS03 Forms, deprecatedkokr
Korean Forms, deprecatedvivn
Vietnamese Forms, deprecatedzhcn
Simplified Chinese Forms, deprecatedzntw
Traditional Chinese Forms, deprecated