With current trunk, whenever you open an IndexReader on a directory you get back a DirectoryReader which is a composite reader. The interface of IndexReader has now lots of methods that simply throw UOE (in fact more than 50% of all methods that are commonly used ones are unuseable now). This confuses users and makes the API hard to understand.
This issue should split "atomic readers" from "reader collections" with a separate API. After that, you are no longer able, to get TermsEnum without wrapping from those composite readers. We currently have helper classes for wrapping (SlowMultiReaderWrapper - please rename, the name is really ugly; or Multi*), those should be retrofitted to implement the correct classes (SlowMultiReaderWrapper would be an atomic reader but takes a composite reader as ctor param, maybe it could also simply take a List<AtomicReader>). In my opinion, maybe composite readers could implement some collection APIs and also have the ReaderUtil method directly built in (possibly as a "view" in the util.Collection sense). In general composite readers do not really need to look like the previous IndexReaders, they could simply be a "collection" of SegmentReaders with some functionality like reopen.
On the other side, atomic readers do not need reopen logic anymore? When a segment changes, you need a new atomic reader? - maybe because of deletions thats not the best idea, but we should investigate. Maybe make the whole reopen logic simplier to use (ast least on the collection reader level).
We should decide about good names, i have no preference at the moment.