First, I'd like to note that my knowledge on English verse is not as good as of Norse. Thus I will start with a description of Norse verse, and then try to compare with what I know of English verse.
Since Tolkien is obviously talking about the verse found in eddic poetry (as opposed to the kind of scaldic poetry that where aimed at Kings and trying to flatter them), I will limit myself to that. The two main verse types in the Poetic Edda are Fornyrðislag ('past-words-form') and Ljóðaháttr ('song metre').
Fornyrðislag is the older form, used in the Edda for epic poetry (such as Völuspá). It is divided into strophes of usually eight lines, with two stressed syllables in each line, and a number of unstressed syllables, which can vary more freely. Alliteration is used to bind two lines together.
Ljóðaháttr is used for dialogue and poetry meant to divulge wisdom. Every strophe consists of two pairs of three lines, where the first and second usually contain two stressed syllables each and are connected by alliteration. The third line contains an alliteration and three stressed syllables. Again, the number of unstressed syllables is freer, and they can be reduced so that they are almost non-existent. Let's take what is probably the most famous strophe in Hávamál as an example, where I have marked the alliterations:
deyr sjalfr it sama,
ek veit einn,
at aldrei deyr:
dómr um dauðan hvern.
(All vowels alliterate with each other.)
It should be noted that while kennings are sometimes used in both forms, they are less common than in scaldic poetry, and when used, less complex.
Old English verse
Tolkien takes Beowulf as his prime example of Old English poetry, which is good for me as it is the work I know best.
The verse in Beowulf has a metre that as far as I can tell is very close to Fornyrðislag, except:
- There are no strophes, but rather just line upon line like in hexameter epics.
- The paired lines in Fornyrðislag are set on the same line with a caesura between, but otherwise work much the same, except that the first half sometimes only contains one stressed syllable.
So, the metre is very similar, but Norse poetry try to condense everything more: where Beowulf have no strophes, Norse poetry have very short strophes which are all usually dealing with one specific topic.
There is also another difference on a larger scale: Beowulf is one continuous poem, with no subdivisions. The closest we can come to something on the same scale in the Poetic Edda is the second part, dealing with the matter of Sigurd. While much shorter in total, and using Fornyrðislag and its strophes, it is also divided on several different distinct poems, each dealing with some distinct story (i.e. The short lay of Sigurd starts when he has already killed Fafnir).
Thus, Norse poetry is, as Tolkien suggested, much more concentrated, both in subject matter and language. It does try harder to make a direct impact, whereas Old English poetry is comparably more sedate, trying to spin a longer story, where the tension is built slower.
- I checked J.R.R. Tolkien's The Legend of Sigurd and Gudrún, just to make sure exactly what poems he was comparing and that I was not missing anything.
- I used Lars Lönnroth's Den poetiska Eddan and his foreword for the parts on the Eddic poetry.
- I also double checked Háttatal in Snorres Edda, translated by Karl G. Johansson and Mats Malm (but that was actually not necessary, as he only gives examples of the two types of verse, and does not discuss them at all).
- For Beowulf, apart from Tolkien, I checked the foreword of my copy of Seamus Heaney's translation, which generally confirmed the above, but I did not find any detailed discussion of the metre.